Discrete Preamp, No IC's !

[Cyber_Murphy]

Hi There,

Just wanted to post my new project,, a ‘Discrete Op Amp’ preamp! No IC’s to be seen in this project

I had a couple of Burson Discrete Op Amp modules lying around un-used. I had tried to use them in another project without success! I tried replacing the LM-4562’s in my modified DCX i/o board, but it didn’t work too well.. Since these modules work best at +/-18V (instead of 15) I had to use a second power supply. It got messy, and had too much noise and hum.. So I scrapped that project

I decided the best approach to best utilise these discrete modules was to build a stand-alone pre-amp. It is based on the Silicon Chip Ultra-LD Preamp, as used in their “Class A†& “Ultra-LD MK3†amplifier projects.

All components used are Good quality: Nichicon, Panasonic, Vishay, Wima, Alps, etc.. The power supply is a “Japanese Kubota design†ultra low noise regulator (does not use IC regulation!) and the Transformer is a 18+18V 30VA custom job from Harbutch with Electrostatic shield & a Flux band..

I tried to keep signal cables to a minimum, and I even used a homemade “volume extender†to avoid having to run signal cables across the case! It’s a bit sloppy,, I’ll need to come up with a better system for the volume extension??

The op amp modules, and the heat sinks on the power supply, get nice and hot that’s for sure!!

I do have a couple of issues though:

1) With the volume all the way down (vol at min) I still get music playing through the speakers?? If I turn up the volume just a bit, then everything goes completely quiet, no sound,, turn a little more, then the volume slowly starts to increase as normal..

2) With the volume turned up to 3 quarters or more (with no music playing) I can hear hum through the speakers. If I touch the shield around the pre-amp (see photos) the hum completely goes away. Strangely enough, with the volume at 100% Max, the hum is much quieter than it is at 75%..??

Any feedback with these 2 points much appreciated

Please excuse the case,, it's very DIY

Cheers..

CM

[Cyber_Murphy]

So.. How does it sound??

**Using it as a standalone preamp (source -> Preamp -> Power amps)

Sounds quite good! Nice clear highs and ‘defined’ lows.. Thick, robust, and realistic sound. Not thin and overly bright... Smooth and non fatiguing! When using my digital source, I have the added bonus whereby the upper-mid “Tizziness†is removed. Currently, with my digital source, I sometimes use the “Jkeny inspired Cheater plug†on my music PC,, which also has the effect of removing that hard edge (Tizziness?) associated with my digital feed..

**Using it as a ‘line buffer’ with volume bypassed (set to max): Digital Source-> DCX(DAC)-> “line buffer†-> Power amps

In this combination it has a really nice effect.. I suppose a DIY version of Burson’s discrete line-buffer?? Smooths out the sound nicely,, instruments and vocals sound more realistic, I can hear more ‘carry’ (decay?),, lovely!!

Removes the bright/harsh edge that I can get from my digital feed (music PC-> jPlay->MK3 Hiface->DCX).

Don’t get me wrong,, I love my Didden i/o mod on the DCX,, but having this discrete circuit in between the DCX and power amps has a positive effect on SQ.. Seems to ‘cancel out’ the by-products of my digital feed..

One thing of note is that the noise floor isn’t as low in my system with this unit.. My system is usually dead quiet, but with this discrete line stage in the loop, it’s not as quiet.. Not too noticeable during normal playback, but in between songs or at low volumes I can tell it’s not as quiet as what I’m used to.

I realise that discrete op amps don’t offer as much PSRR as monolithic IC op amps, but if I can solve that hum issue as mentioned in my opening post, I think this will improve the “quietnessâ€..

Thanks

CM

[RoHo]

Nice build, Cyber. Those "discrete op-amps" are intriguing things.

In response to your issues, I reckon:

1) Dodgy volume pot. Either replace it or just don't turn it all the way down. Or maybe.....see below

2) Could be a grounding issue and these can be mongrels to track down. I notice that the pot is soldered directly on to the PCB. If it's not attached to the chassis I would try shorting the metal thread/body of the pot to the chassis somewhere.

[Cyber_Murphy]

Hi RoHo, Thanks for your feedback.. They are definately nice sounding, but tricky to get right!

The volume is an Alps Blue velvet,, so I shan't think it is faulty?? It measures OK with a DMM, ie; it goes down to zero (between wiper and pin1 (GND) when fully at min, and 10K when turned up all the way..

I'd say a ground issue.. These Discrete modules sure are a 'noise magnet' if you're not careful! Yes, the body of the volume pot is connected to Earth, via the rear screw that holds the front in place (see below),, there is continuity between the volume shaft and Earth..

Ta

CM

Edited September 5, 2012 by Cyber_Murphy

[RoHo]

So no easy fix then! Re the pot, the wiper may be slightly mis-aligned with the end of the track, but it's not a big deal.

As I said, grounding issues are a pain. I may be telling you how to suck eggs but do you have a star ground? This is not so simple when you connect a few PCBs together. Where do the ground / 0V points from each board connect back to Earth?

[Addicted to music]

CM,

is that a pastic case you are placing it in?

I have built the Silicon Chip ULD opamp issued around 2004. I did it in 2010 so I can have a listen to the SQ of opamps and to check what other opamps are like when I rolled them in. I also modified some of the pcb to reduce components and experiment with the different MKPs out there.

If I where to tackle this project I would have built it to Silicon Chip's specifications to ensure it works and then swopt the opamps, if that worked then I would move to the next step and replaced the psu etc. Get my drift?

Star earthing as pointed RoHo is also important thats why I have zero in on the plastic case!!!

[Cyber_Murphy]

Ok, just finished a solid listening session!

I'm very satisfied, it sounds really good,, I enjoy the sound of these discrete op-amp modules.. Either as a pre-amp for my Analog source or as a "line buffer" for my diigtal feed, the sound is detailed but smooth..

In normal listening, I cannot even hear any humming, only in between songs.. Increased noise floor is not evidant when used as a line buffer with volume at max, although when used as a stand alone pre-amp, its not as 'quiet' as my dcx, which has LM4562's..

@Roho, yes star ground is in use, all grounds go to the same piont, then a single wire from there goes to the Earth connection on the power connector.

@Pchan, thanks for sharing your photos.. Yes, I did build it to SC specs,, I even put LM4562's in before the discrete modules to make sure it worked,, no hum with IC's, but I don't think IC (monolithic) op-amps would ever hum anyway, their PSRR is way too good! Strangely enough I still have that issue with the IC op-amps where I can hear music with volume all the way down?? Oh by the way, yes plastic case but with a steel shield / separator around the pre amp board.. You mention power supply?? I am using a "Kubota regulator", very good!! Much better than LM317 / LM337 type power supply :-)

Ta

CM

[Addicted to music]

HI CM FWIW,

with the volume issue try 20K if you have one lying around, with the hum issue I think the transformer is too close to the volume pot, you have the volume shaft sticking out through the shield that acts like an antenna for hum. If you check on the photo that i posted look at the distance of the transformer and the main audio pcb. Note also that its shielded away from the psu and all pcbs. With any opamp module the gnd is not present, only the -ve and +ve supply rail are introduced, therefore those modules are like you said an antenna for hum and RF noise.

Also Silicon chip introduced caps on either side of the pot to eliminate DC so no static can be heard when rotation the pot. Although Im not sure if they have done it to the module you have. I bypassed one of the cap ( the output of the 1st opamp) and that took it to another level of SQ. I also bypassed the input cap as most sources will have an o/put cap and no DC is ever present. This move raised the level of SQ again. I then replace the compulsary caps from electrlytic to MKPs and it smoothed out edginess. You can see the photo where I used the same values as 47uf MKPs are huge so i wired it of the pcb.

I then started opamp rolling and I found the LME 49720 and the ADA 4627-BRZ to be the best sounding IC opamps, I stopped short in ordering Bursons, too expensive and I already have a discrete pre already.

I notice youre in VIctoria, if you are in the Western suburbs PM me to drop in for a listen! I also inherent a 30yr old CRO if you ever need one!

Edited September 5, 2012 by pchan

[andyr]

... and the Transformer is a 18+18V 30VA custom job from Harbutch with Electrostatic shield & a Flux band..

Can I suggest you try a 180VA transformer (in place of a 30VA) - even though you are not drawing anywhere near that current.

I tried this on an active XO (which also has a tiny power consumption), on the advice of a knowledgable mate ... and was surprised to hear that I got a lot more bass, as a result! :eek:

Regards,

Andy

[Cyber_Murphy]

HI CM FWIW,

with the volume issue try 20K if you have one lying around, with the hum issue I think the transformer is too close to the volume pot, you have the volume shaft sticking out through the shield that acts like an antenna for hum. If you check on the photo that i posted look at the distance of the transformer and the main audio pcb. Note also that its shielded away from the psu and all pcbs. With any opamp module the gnd is not present, only the -ve and +ve supply rail are introduced, therefore those modules are like you said an antenna for hum and RF noise.

Also Silicon chip introduced caps on either side of the pot to eliminate DC so no static can be heard when rotation the pot. Although Im not sure if they have done it to the module you have. I bypassed one of the cap ( the output of the 1st opamp) and that took it to another level of SQ. I also bypassed the input cap as most sources will have an o/put cap and no DC is ever present. This move raised the level of SQ again. I then replace the compulsary caps from electrlytic to MKPs and it smoothed out edginess. You can see the photo where I used the same values as 47uf MKPs are huge so i wired it of the pcb.

I then started opamp rolling and I found the LME 49720 and the ADA 4627-BRZ to be the best sounding IC opamps, I stopped short in ordering Bursons, too expensive and I already have a discrete pre already.

I notice youre in VIctoria, if you are in the Western suburbs PM me to drop in for a listen! I also inherent a 30yr old CRO if you ever need one!

Hi Pchan,

Thanks for an interesting post..

RE the 20K pot?? Silicon Chip comment in the Nov 2012 issue (Ultra LD MK3 "revamp" of the this pre-amp), specifically they mention the if they could use a 4.7K pot (instead of 20K) they would have, as it reduces THD+N even further,, so to compensate they parallel a 4.7K resistor.. My spare blue velvet was 10K and I used a 10K in parallel to bring overall value (at full vol) to 5K,, close to SC's ideal of 4.7K.. But you suggest to go back to their original design of using a 20K..??

The volume body/shaft is Earthed, and the transformer has a GOSS fulx band and Electrostatic shield which is also Earthed.. I figured this was more than enough protection from noise pickup!! I intentionally wanted a "smallish" enclosure, hence everything is closely mounted..

RE bypassing the cap on the output of Op amp 1?? Do you mean bypassing this one:

I'd rather leave the cap on the input.. All audio path coupling cap are Muse ES..

The circuit doesnt have any 47uF Electrolytics to replace?? Which "compulsary" ones did you replace?? Decoupling caps I have used are wima MKPs and Electros are Panasonic FM (the big Grey 100nF MKP across pin 4 & 8 is Vishay!)

And thanks for your offer to Demo you pre-amp!! Although I am in the South East, perhaps one day I will make the journey :-)

Ta

CM

Edited September 6, 2012 by Cyber_Murphy

[Cyber_Murphy]

Can I suggest you try a 180VA transformer (in place of a 30VA) - even though you are not drawing anywhere near that current.

I tried this on an active XO (which also has a tiny power consumption), on the advice of a knowledgable mate ... and was surprised to hear that I got a lot more bass, as a result! :eek:

Regards,

Andy

Interesting concept.. I waited 8 weeks to get this transfomrer from Harbutch.. Don't think I could go thru that again just to try a 180VA

Plus, that would be too big?? With about 7 to 10 % regulation, those heatsinks would get really hot (series pass mosfets),, they already do..

Have a close look at the PSU in the photos,, It's got plenty of good quality filtering,, this "Kubota regulator" is a beast!!

Ta

CM

Edited September 6, 2012 by Cyber_Murphy

[andyr]

Interesting concept.. I waited 8 weeks to get this transfomrer from Harbutch.. Don't think I could go thru that again just to try a 180VA

Plus, that would be too big?? With about 7 to 10 % regulation, those heatsinks would get really hot (series pass mosfets),, they already do..

Have a close look at the PSU in the photos,, It's got plenty of good quality filtering,, this "Kubota regulator" is a beast!!

Ta

CM

Just because it's a 180VA transformer doesn't mean more current is going to flow (so heatsinks will get hotter than they are at present).

The point I am trying to make is that (and this goes against a "normal" train of thought) bass is better when you use a grossly overspecced (in VA terms) power transformer. IOW having a gross amount of headroom, in terms of the current being drawn through the transformer vs. what it is capable of ... delivers better bass. I wouldn't have believed this myself - except I actually tried it out! And the only reason I tried it out was because I respected the opinion of the the guy that told me about this "trick".

Regards,

Andy

[Cyber_Murphy]

Thanks Andy,, I'll hunt around for one in my spares and give it a try!!

What I was trying to get at was, the load on a 180VA in this circuit would be low, keeping the secondary Voltage higher than a 30VA (at the uusal transformer regulation of about 10%), hence the regulator will need to dissipate more heat to keep a regulated output at my required output voltage.. What I probbaly assumed (incorrectly) was you meant an 18+18V 180VA transformer,, but at 180VA I would propbably be better off with a 16+16V rating..

Either way,, the only limiting factor with the Kubota is the size of the heatsink,, so your reccomendation would easily be possible with a bigger heat sink

Thanks..

CM

[Addicted to music]

Hi CM,

thnx for posting the part of the diagram, the one that I built is a little different to your configuration. SC love reducing the value on those pots to chase academic figures. Just wanted to asked that the second opamp is unity gain?, that is a gain with feedback configured so its 1. You then have to ensure that the Burson modules you are using are within its capacity to do this. There are lots of IC opamps that are not designed for unit gain. Not sure whether the Bursons are designed for this but then again you have said that it all works, so it shouldnt be a problem until you get hum and still volume at the lowest levels.Check this before you get to carried away in changing the pot.

Yes, looking at the circuit the 22uf is the one I would attempt to bypass. Only performed any of my suggestion if you are comfortable in doing so! I bypassed the input cap because I know that most sources have an output cap or a DC servo for zero DC out. So it is compulsary to leave the o/put cap in circuit for safety, as this is your last physical line of defence in case the whole thing goes faulty and swings to one rail!

FWIW, it is great to see Andy provide the suggestion on the transformer. But I have read somewhere, a post that ZB had said that with these low voltages it is better to use a laminated transformer or a c/core as this has better utilisation of the windings at low levels. This is possibly why you see Audio GD uses C/core transformers and not toroidals. Toroidals are great for higher voltages as they become more efficient at this level. This possibly explained the effects when I went from laminated to toroidal and the SQ was veiled in a previous full discrete preamp I built but I could never understand why!

[Cyber_Murphy]

Hi Pchan,

Ok, I will try bypasisng the cap on the output of op-amp 1.. I'm thinking of getting the "direct out" DAC from hifimediy (CS4398) which has some nice Mundorf MCAPs on the output,, if I do, then I will also try bypassing the input cap too!

Yes the second op amp is set for unity gain, and yes, the Burson modules are fine operating at unity..

I'm sitting hear listening at the moment, and this pre-amp sure does sound sweet!! Brings realism and body that I haven't been used to with my usual LM4562 / CS3318 output combo,, which although also sounds good, can sometimes become too sterile and "clean", especially with my digital feed..

If I hover my hand close above the circuit boards, I get a funny whistling sound,, a bit like a theremin :-)

I think I will mount this in a larger, metal or aluminium case, and as you suggest, keep the transformer a bit further away from the main board..

Ta

CM

[Addicted to music]

Hi Pchan,

Ok, I will try bypasisng the cap on the output of op-amp 1.. I'm thinking of getting the "direct out" DAC from hifimediy (CS4398) which has some nice Mundorf MCAPs on the output,, if I do, then I will also try bypassing the input cap too!

Yes the second op amp is set for unity gain, and yes, the Burson modules are fine operating at unity..

I'm sitting hear listening at the moment, and this pre-amp sure does sound sweet!! Brings realism and body that I haven't been used to with my usual LM4562 / CS3318 output combo,, which although also sounds good, can sometimes become too sterile and "clean", especially with my digital feed..

If I hover my hand close above the circuit boards, I get a funny whistling sound,, a bit like a theremin :-)

I think I will mount this in a larger, metal or aluminium case, and as you suggest, keep the transformer a bit further away from the main board..

Ta

CM

CM, if you are considering getting that CS4398, think twice about bypassing those Mundorf caps as the the DAC o/put has a 2-2.5V DC, thats why they are used! Unless you know how to place a DC servo circuit to reduced this offset, but at 2.0V out it will be hard to do.

SC do discuss in detail about component layout and placement and I think that I read in the article of there latest offering; the preamp pcb you selected, explains how one of the power amp channel had worst noise figures, this was due to the wiring being too close to the transformer and psu assembly. They then employed a shield around the toroidal used.

I also had a lot of fun with opamp rolling in the unit I built. The feeling you get when one of these project are completed and up and running is undescribable. I reckon its better than sex, but lets keep that our secret as my better 1/2 just wont understand.

[Cyber_Murphy]

Hi Pchan,

I think SC had initially Spec'd a shielded transformer, which of course, Altronics did nto want to provide,, so they eneded up doing their own 'sheild' by wrapping copper around it.. I also think this was about transformer noise bleeding directly into the power-amp board and not so much the pre-amp side.. Nevertheless you are correct, it was due to the transformer being to close!!

Like I said, my transformer has electrostatic shield aorund the primaries, and a Flux band wrapped around the secondaries,, I would've thought that was enough,, but evidentally it's not.. I will mount the project in a larger aluminium case and keep transformer further away.. Sigghh,, I hate case work :-(

**By the way, if I go with the hifimediy DAC kit, I meant I will bypass the 22uF on the input of the pre-amp, because the DAC already has nice Mundorf caps on its output..

Thanks..

CM

*Edit: one more thing re op-amp rolling,, if i swap LM4562 in place of Burson modules,, sound goes thin, and looses its "relaism and body"

Edited September 6, 2012 by Cyber_Murphy

[Addicted to music]

Hi CM,

can you direct me to the site where you got the cs3310 please! I happen to like these because Im going to recap and redo my discrete preamp I already have.

I like to here the Bursons on day but Im strapped for cash ATM. There is another set called DEXA thats available through parts connection and then theres the Audio GD opamps but I would take it you already know about these.

Compared to the Bursons do you think that the LM4562 may not be as full body as you say but do you thin its value for the cash? I cant comment because I've never have heard them.

[Cyber_Murphy]

Hi pchan,

Just a quick response as i'm on the mobile..

Site is hifimediy.com Lots of info on diyAudio about this DAC..

Once i re-case my unit am happy to bring it over so u can listen. i use sockets, so easy to pull them out too...

Ta

CM

[Cyber_Murphy]

Hi Pchan,,

I just realised your post refers to where I got the CS3310 from...?? I thought you were asking where I plan to get the "direct out" CS4398 DAC from,, which is from here..

I dont have anything with CS3310.. My DAC/Pre is a modified DCX which uses a CS3318 for 8 channel volume control (2 input, 6 output).

The chip was part of the "Jan Didden active i/o kit" which comes with the CS3318 pre-soldered on the circuit board (Nominal pin pitch is 0.50 mm !!)

Thanks!

CM

Edited September 8, 2012 by Cyber_Murphy

[Addicted to music]

Hi Pchan,,

I just realised your post refers to where I got the CS3310 from...?? I thought you were asking where I plan to get the "direct out" CS4398 DAC from,, which is from here..

I dont have anything with CS3310.. My DAC/Pre is a modified DCX which uses a CS3318 for 8 channel volume control (2 input, 6 output).

The chip was part of the "Jan Didden active i/o kit" which comes with the CS3318 pre-soldered on the circuit board (Nominal pin pitch is 0.50 mm !!)

Thanks!

CM

OK, so this DAC/pre kit is no longer available!

[Cyber_Murphy]

Hi,

Yes, it's still available.. You can read about it here, And also from here

Ta

CM

[andyr]

Thanks Andy,, I'll hunt around for one in my spares and give it a try!!

What I was trying to get at was, the load on a 180VA in this circuit would be low, keeping the secondary Voltage higher than a 30VA (at the uusal transformer regulation of about 10%), hence the regulator will need to dissipate more heat to keep a regulated output at my required output voltage.. What I probably assumed (incorrectly) was you meant an 18+18V 180VA transformer, but at 180VA I would propbably be better off with a 16+16V rating.

Either way, the only limiting factor with the Kubota is the size of the heatsink, so your reccomendation would easily be possible with a bigger heat sink

Thanks.

CM

CM, I cannot comment as to whether 16v secondaries would be all you need, if you substituted a much larger trannie - I simply used the same secondary voltage for the higher VA version.

Regards,

Andy

[andyr]

FWIW, it is great to see Andy provide the suggestion on the transformer. But I have read somewhere, a post that ZB had said that with these low voltages it is better to use a laminated transformer or a c/core as this has better utilisation of the windings at low levels.

Hi Peter,

Did perhaps ZB say "at low currents" ... not "low (secondary) voltages"? The reason I ask is because AIUI, the only reason to use toroids is that they have a much smaller magnetic field compared to a "conventional" transformer - so in a power amp when amps are flowing through the power trannie ... this is a good thing! But a source component has a small current - so it is more important to be as efficient as possible?

ZB, where R U!

Regards,

Andy

[Addicted to music]

Hi Peter,

Did perhaps ZB say "at low currents" ... not "low (secondary) voltages"? The reason I ask is because AIUI, the only reason to use toroids is that they have a much smaller magnetic field compared to a "conventional" transformer - so in a power amp when amps are flowing through the power trannie ... this is a good thing! But a source component has a small current - so it is more important to be as efficient as possible?

ZB, where R U!

Regards,

Andy

Andy,

send ZB a PM and you get it straight from the horses mouth!

Im sure but I could be wrong like I always am in cases like this, that at low voltages (0-20VDC) it is better to use laminated or a c/core as the windings are better utilised. This is why you see Audio gd and many others use them for DACs and low voltage applications. Normally when you have 0-20VDC its all low level stuff anyway and not current driven like a power amp. I remember ZB posting that toridals are good for high current , high voltage as the are more efficient and produced less heat.

Regards

pchan