Proud DIY amplifier and speaker builder

[roger777]

Hi,

I've been constructing amplifiers and speakers for my own home use as a hobby for decades. Back in the day I even used my EA Series 2 mosfet amp and two pairs of my speakers for a dance in the student hall of residence I was living in at the time. That amplifier was barely powerful enough, but it was all that was available on that particular night.

During the last three months I replaced the drivers as an upgrade in those same two pairs of cabinets (again) and very carefully designed the crossovers using the free software available online. Using this software certainly beats using a spreadsheet or manually calculating impedance and response magnitude and phase values at half octave intervals.  I think that both resulting speaker configurations in those cabinets are the best speakers I  have had. The smaller 17 litre two-way pair that still had its original Philips drivers from the 1980's is a particular relative improvement.

I also recently assembled a 200 watt Class D amplifier using off the shelf pre-assembled circuit boards. Again this is definitely the best amp I have owned, albeit not the most powerful. I think that honour belongs to my Playmaster sub woofer amp.

I am about to repair and/or improve my Playmaster Pro Series 4 preamp. It is long in the tooth, possibly has the odd dry solder joint, worn out switch, and failed potentiometer. Unfortunately the switches and potentiometers are spline shaft, which so far I have been unable to source.  I replaced the phono preamp board years ago because the original circuit was unstable. However, the kitset circuit I replaced it with clearly had a design pretty much taken directly from the LM833 IC application notes, and its distortion was too high for my preference. I fixed this by lowering the overall gain by 7dB. I'm in the middle of working on a design that has all of stability, low distortion and sufficient gain.

I also have an old ETI Series 5000 amplifier and preamp kicking around. Unlike some people on this forum, I don't consider the amp worth repairing. It still works but does not sound too good. The Class D amp I assembled is better in every respect than a Series 5000 amp performing to spec. I still use the Series 5000 preamp occasionally, but it doesn't sound fabulous and the other year I replaced all the tantalum capacitors because I found many of them had failed and gone closed circuit. Goodness knows how the voltage regulators survived, but they did.

By the way, I carefully only mentioned by name brand products that don't exist anymore. I assume it is okay to discuss "live" brand products, but I omitted the names in the unlikely event that including them may beach a site rule in thi brief introduction.  My employment is unrelated to electronics.

 

[Sub Sonic]

Welcome aboard, you will be in good company here

 

Could you post a couple of pics (and specs) of the switches and pots you are after? I can have a look through a collection of parts I have at home - postage across the ditch wouldn’t be much.

 

Cheers,

 

Matt.

[roger777]

Thanks for your helpful offer Matt. Google was no help in locating those parts from anywhere. Right after posting my introduction above it occurred to me to try Ebay Australia. To my pleasant surprise it has numerous suppliers of those parts. I placed orders last night.

The parts I was looking for were a 2 pole six position rotary switch and a B100K dual gang mini potentiometer, both of the spline shaft type as used to be sold by DSE, but not Jaycar and Parts Express. It seems they are only manufactured in China. I also found a few on Alibaba just now, but its search function is not as good or it provides fewer options.

[Sub Sonic]

I’ll have a look at home in the next couple of days in case I can find anything - might give you some more options

 

The (large) collection of parts was my late father’s - he was an electronics engineer with an interest in audio, so there may be something.

 

Cheers,

 

Matt.

[Sub Sonic]

I had a look but only found a mini A50k dual gang spline pot.

 

Any other odd parts you find yourself looking for, feel free to sing out

 

Matt.

[roger777]

9 hours ago, Sub Sonic said:

I had a look but only found a mini A50k dual gang spline pot.

 

Any other odd parts you find yourself looking for, feel free to sing out

 

Matt.

Thanks Matt. Much appreciated. I don't need an A50k pot at this time. Cheers.

Edited March 1, 2021 by roger777
Confirming I have no need for A50k pot.

[Monkeyboi]

On 28/02/2021 at 6:53 PM, roger777 said:

I still use the Series 5000 preamp occasionally, but it doesn't sound fabulous and the other year I replaced all the tantalum capacitors because I found many of them had failed and gone closed circuit. Goodness knows how the voltage regulators survived, but they did.

 

Failing tantalum capacitors were my same observations with the ETI 5000 series Control Preamplifier   I have built a couple of these.  The first one was lost in a house robbery and I came upon a partially assembled kit many decades later on Ebay.  The 2nd kit functioned well for about six months before the power supply decoupling tantalums (the circa 1980s ones supplied with the kit) started going short circuit. Strangely taking them out of circuit and testing them with a LCR meter they appeared to be normal however leakage testing them revealed otherwise with them failing before the rated voltage.  The solution is simply to replace them with ones with a slightly higher working voltage.

 

Improvements can be made to the original David Tillbrook design.  In my case I built the second preamp using IC sockets on the mainboard.  This alone doesn't improve SQ but allows for limited IC rolling with pin for pin compatible OP amps.  I eventually settled for the NE5534AN low noise versions as used in the phono stages.

 

Among other improvements were to replace the coupling electrolytics in the audio signal path with Elna Silmic II caps, replace the monitor and master level potentiometers  with ALPS Blue units, open cage trim pots with sealed units and upgrade the  capacitors in the power supply filtering.

 

Considering the ETI Series 5000 gear is now 40 years old, IMHO the most likely culprits that are contributing to the somewhat "tired" sound is undoubtedly the tired 40 year old aluminium electrolytic and tantalum capacitors.

 

Here's the link to the forum discussion on the ETI 5000 Series Control Preamplifier if you're interested.

Resurrecting a 1980s analogue preamp project - DIY Audio Projects - StereoNET

 

 

Cheers,

Alan R.

 

   

Edited March 2, 2021 by Monkeyboi
added forum link

[roger777]

5 hours ago, Monkeyboi said:

 

Failing tantalum capacitors were my same observations with the ETI 5000 series Control Preamplifier

 

   

Thanks Alan. I understand that the issue with the Tantalums is both an age thing and it is possible they were not as well manufactured in 1981. I am sure many of the modern devices with tantalum capacitors (phones etc.) will be completely useless even ten years from now for reasons other than capacitor failure.

I used plain electrolytics for my fix. Suffice to say I did it in a hurry. My reason for opening it up in the first place was actually that in storage it had been sprayed by a cat!

Thanks for your link to the Series 5000 pre-amp discussion thread. There are some very useful insights there. It is a long time since I considered imbalance in ganged potentiometers, and this may be a worsening problem with the Playmaster Pro Series 4 Preamplifier that I currently use. I also have the common problem that I have not managed to get the LED VU meters in the Series 5000 pre-amp to match.

Edited March 2, 2021 by roger777
Corrected Playmaster preamplifier name.

[Monkeyboi]

Hi Roger,

Yes, those extended range VU meters, whilst it's a great idea implementing both peak and average in the one display aren't completely without their issues.  If you can't get the two meters to track accurately check the audio input to the VU meters is the same on each board for various positions of the master level control.  If the signal input voltages are the same, adjustment of the input sensitivity is achieved by trimpot RV1.

 

If you don't have access to the ETI magazine article instructions on how to set up the VU meters the following may be of assistance.

 

LED Level Meter Adjustment (Calibration set-up) for each board

 

 

Underside (printed wiring side) of LED Level Meter board

 

1.      Ensure audio input to the board is short circuited to ground.  That is zero input signal.

2.      Adjust both RV2 and RV3 trimpots to the fully anti-clockwise position as viewed from the underside (printed wiring side) of the PCB.

3.      Adjust RV3 slowly in a clockwise direction until the second LED (-45dB indication) has just illuminated.  This is the peak level indication LED adjustment.

4.      Adjust RV2 slowly in a clockwise direction until the average level LED adjustment is such that this illuminated LED is superimposed on the peak LED adjustment at the -45dB indication.

5.      Now readjust RV3 so that both the peak and average LED indications have just run off below the lowest level (-48dB indication) on the display.  With no input signal present, no LEDs should be illuminated.

6.      Now remove the short circuit from the input to the meter board so that the level indications can be calibrated using steps 7 onwards.

7.      Apply a sine wave signal of 1kHz to the input under test.  This can be done through one of the pre-amplifier high level inputs if desired.

8.      Set the Balance control to mechanical centre.

9.      Connect a true RMS voltmeter or oscilloscope to the LINE output of the pre-amplifier and adjust the MASTER level control on the pre-amplifier until a signal level of 2.0 VRMS is obtained at the LINE output socket for the channel under test.

10.   Now adjust RV1 trimpot on the LED Meter board under test so that the peak LED illuminates the 0dB (orange) LED.
The calibration adjustment is now complete.

11.   Vary the input signal level to check that as the level is varied from minimum to maximum that all the LEDs in the board illuminate according to the input signal level.

Repeat the adjustment steps 1 through to 11 for the other channel affecting adjustments on the LED Level Meter board for that channel.

NOTE: If performing the initial calibration adjustments during amplifier assembly it is recommended that the right channel meter be adjusted first as access the trimpots RV1, RV2 and RV3 on that board is impossible once the left channel meter board has been installed above it within the chassis.  If the procedure has to be repeated on the right channel board, the left channel board will have to be moved out of the way to effect this.

 

Cheers,

Alan R.