Monkeyboi Posted November 6, 2020 Share Posted November 6, 2020 On 05/11/2020 at 11:53 AM, Leinster Lad said: And what has the voltage rating got to do with it. EG, C1 , the input cap on the VU meter board is specified as 1 uF @ 6v ..... can't find one. But I can find a 1uF in 10v and a 16v and a 25v .... Others are spec'd at either 16v or 25v . Is there any actual technical reason ? Why not just spec all of them at 25v ? gives a good margin over the supply rail voltages and would simplify the BOM, surely ? Okay, the voltage ratings specified on the BOM for the tantalums used in the 5000 Series Pre-amp VU meters were based on cost, availability and physical size at the time of the design. Note the capacitors are mounted on their sides to facilitate the close proximity installation of both VU boards the original case. Standard low leakage (LL) aluminium electrolytic capacitors available at the time just wouldn't have fitted on the board. Fortunately, tantalum capacitors like many electronic components have shrunk in size over the decades since this pre-amp was originally designed, so using tantalums with a higher voltage rating is not an issue in this application. However I wouldn't use anything under 25v rating in this circuit as 16v IMO is sailing a bit too close to the wind with 15v supply rails. Tantalums are often used in circuits that require low leakage currents and where the capacitor has to be small in physical size for the capacitance and voltage rating compared to say conventional aluminium electrolytics or LL types. As I commented earlier, IMHO and IME, tantalums should be avoided as signal coupling and signal decoupling capacitors in audio circuits, BUT in this case of the VU meters it doesn't matter as the VU level meter boards are only "listening" to the level of the signal and are not part of the audio reproduction chain that you actually hear through the pre-amp. As for other tantalum capacitors used in the 5000 Series pre-amp you will find them in the power supply as bypass capacitors C9, C12 and C13 on the outputs of the voltage regulators. Here, they don't affect audio quality as no signal passes through them and as I stated in an earlier reply tantalums have some desirable characteristics in these roles and I would not use conventional aluminium electrolytics as substitutes as replacements for C9, C12 and C13. Agreed, same voltage rating would simplify the BOM cost if ordering large quantities. Cheers, Alan R. Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 7, 2020 Share Posted November 7, 2020 Thanks Alan this is exactly what I hoped you would say !! Link to comment Share on other sites More sharing options...
Monkeyboi Posted November 7, 2020 Share Posted November 7, 2020 On 05/11/2020 at 11:49 AM, Leinster Lad said: Thanks for the explanation Alan. So, I was looking at Tantalum caps @ element14 to build the BOM for the 5000 pre..... Is there any way that you could explain to me the incredible range and price difference between caps of identical specs (uf, V ) ?? I get different manufactures and to some extent different tolerances but ......... How the heck is anyone able to chose one over the other ??? Manufacturers and distributors marketing. In the case of the VU meters I wouldn't be wasting my money on any "audiophool" capacitors here because no one is going to hear the difference. Only the VU meter board will know and they won't care. Yes, it is sometimes confusing when choosing one manufacturer over another. Not all represent their data / specification in the same format for easy comparison. Some esoteric manufacturers actually appear to purposefully omit the important technical stuff and It can be a headache at times making an informed choice on limited information. If I offer one piece of advice it is to use quality components that are currently in production (I tend to avoid NOS vintage stuff in new designs) that represent good value for money in their intended application without over capitalising. Having a way of tracing the lineage of components is helpful if you are building for commercial sale, repeatability or in reliability critical situations. Just a note on the rule of diminishing returns. Some audiophiles IMHO spend ridiculous amounts of money on so called "audiophile grade components" which in many applications don't return anywhere near a proportional improvement for the investment. Some is just audiophoolery marketing hype and DIY novices often fall victim to the advertising claims which 99.9% of the time can't be backed up with any credible evidence other than "he said, she said...." subjective comment. The stuff a lot of audio reviews are like. "...it was like deer's feet walking across the snow....." = IOW I don't know what to write but hey it will sound good in a review. Another favourite is "warm sounding" = IOW (in the context of triode tube amplification) is another phrase for an audible amount of even (mostly 2nd harmonic) distortion. Is it unpleasant? Not generally but it's still adding something to the signal that wasn't there originally. I could spend hours explaining components and their characteristics but I've got a nice Pioneer F-91 tuner on the bench to repair at the moment. Perhaps I should pull my finger out and find time to put up a YouTube video on the subject? Cheers, Alan R. Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 7, 2020 Share Posted November 7, 2020 (edited) Fortunately I am not rich enough to get sucked into that vortex . Good luck with the tuner Edited November 7, 2020 by Leinster Lad 1 Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 8, 2020 Share Posted November 8, 2020 Hi Alan. Just wondering if you would know what caused a small DC voltage on the speaker terminals ? About 80 mV ? Both channels. ETI477 amp Link to comment Share on other sites More sharing options...
acongdon Posted November 8, 2020 Share Posted November 8, 2020 More buried treasure here too.. now back to that amp.. Link to comment Share on other sites More sharing options...
Monkeyboi Posted November 9, 2020 Share Posted November 9, 2020 (edited) 16 hours ago, Leinster Lad said: Hi Alan. Just wondering if you would know what caused a small DC voltage on the speaker terminals ? About 80 mV ? Both channels. ETI477 amp Just a quick question or two. 1. Does it change as the amplifier warms up? 2. What happens to the d.c. offset when you adjust the bias trimmer pot RV1? Cheers, Alan R. Edited November 9, 2020 by Monkeyboi 1 Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 9, 2020 Share Posted November 9, 2020 Hi Alan, Measurement changed by a mV or two over 10 minutes of warming up. I have not yet tried checking or adjusting the output bias (RV1) I was going to wait for a quiet hour or two and do voltage measurements across various resistors, as noted on the schematic. Depending on what I find, measure and replacing out of spec resistors with 1% metal fims. I was then going to start replacing caps. If that still didn't get it sorted, it will be onto the transistors. Link to comment Share on other sites More sharing options...
Monkeyboi Posted November 9, 2020 Share Posted November 9, 2020 2 minutes ago, Leinster Lad said: Hi Alan, Measurement changed by a mV or two over 10 minutes of warming up. I have not yet tried checking or adjusting the output bias (RV1) I was going to wait for a quiet hour or two and do voltage measurements across various resistors, as noted on the schematic. Depending on what I find, measure and replacing out of spec resistors with 1% metal fims. I was then going to start replacing caps. If that still didn't get it sorted, it will be onto the transistors. I'm confident that adjusting the bias whilst simultaneously monitoring the quiescent current and the d.c. offset at the output will provide some answers. Cheers, Alan R. Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 9, 2020 Share Posted November 9, 2020 Thanks Alan, i'll give it a shot and report back ( input shorted, no speakers connected ) 1 Link to comment Share on other sites More sharing options...
MarcAL Posted November 9, 2020 Author Share Posted November 9, 2020 (edited) 4 hours ago, Leinster Lad said: Thanks Alan, i'll give it a shot and report back ( input shorted, no speakers connected ) Yes that is the go with the input shorted. You will need to identify and document over time both mVAC and mVDC at the output terminals to be able to pin point the problem. I would not change any parts at this stage including capacitors. Have a good look at the wiring runs throughout the amp first and take a picture if you find anything suspect or out of the ordinary. When I rebuilt mine several times over the years cable runs made a big difference with noise. I am not a fan of 240V mains wiring running around any circuit boards so that is why I decided on the very minimalist approach on my last rebuild. I take it that this is the newly acquired but nicely built amplifier, yes? Edited November 9, 2020 by MarcAL Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 9, 2020 Share Posted November 9, 2020 3 minutes ago, MarcAL said: I take it that this is the newly acquired but nicely built amplifier, yes? Correct Marcus. This is the one that was being sold locally to me. And yes, it looks very well build ! I'll check out what is going on on the outputs with my trusty scope. Hope to find some answers there. Keen to measure the voltage drop across the resistor "pairs" as well. That might give me a clue as to what is going on. Link to comment Share on other sites More sharing options...
MarcAL Posted November 9, 2020 Author Share Posted November 9, 2020 14 minutes ago, Leinster Lad said: Correct Marcus. This is the one that was being sold locally to me. And yes, it looks very well build ! I'll check out what is going on on the outputs with my trusty scope. Hope to find some answers there. Keen to measure the voltage drop across the resistor "pairs" as well. That might give me a clue as to what is going on. Which pairs, the differential pairs? Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 9, 2020 Share Posted November 9, 2020 (edited) 36 minutes ago, MarcAL said: I take it that this is the newly acquired but nicely built amplifier, yes? Resistor pairs would be R9 and R14 R7 and R18 R10 and R13 The actual exact voltage needs to be very close to specified, but they need to be as close as possible to being the same ! I'll be checking the voltage drop across R11, R12, R15, R17, D1/D2 etc as per the schematic. There will be clues in there ! Edited November 9, 2020 by Leinster Lad Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 On 09/11/2020 at 9:25 AM, Monkeyboi said: Just a quick question or two. 1. Does it change as the amplifier warms up? 2. What happens to the d.c. offset when you adjust the bias trimmer pot RV1? Cheers, Alan R. Alan playing with the output stage bias now. No change at all to the dc offset voltage on the speaker terminals Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 Had a bit of a measure around the various resistors and voltages are damn near exactly as specified on the schematic Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 Offset voltage on the speaker terminals is about 0.07 volts and it is negative on both channels. I have set quiescent current to 170 mA. So about 85mA thru each output pair. Main heatsink sitting on 42 degrees Link to comment Share on other sites More sharing options...
andyr Posted November 11, 2020 Share Posted November 11, 2020 3 minutes ago, Leinster Lad said: Offset voltage on the speaker terminals is about 0.07 volts and it is negative on both channels. I have set quiescent current to 170 mA. So about 85mA thru each output pair. Main heatsink sitting on 42 degrees 70mV? Won't do any harm but it would be noice to get it down to, say +/-10mV? My (very limited!) understanding is that it's a resistor value which sets the offset. In the case of the amps which I am currently building, the pcb designer used a very neat trick! He used a connector in parallel with a pair of holes which take the final resistor. You first of all insert the 3 legs of a Bourne pot into the connector ... adjust the pot for zero offset - and then remove it and insert the final resistor having the pot value. Andy Link to comment Share on other sites More sharing options...
MarcAL Posted November 11, 2020 Author Share Posted November 11, 2020 Is R8 10K or 22K? Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 Replaced the bc550’s and the offset voltage is now about 50mV Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 13 minutes ago, MarcAL said: Is R8 10K or 22K? 10k Link to comment Share on other sites More sharing options...
MarcAL Posted November 11, 2020 Author Share Posted November 11, 2020 For some reason mine are 22K but I can't remember why that was changed from 10K. The BC550's in the differential pair will make a difference from memory. Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 The bc550’s that came out were 2 different types / manufacturers. 5 of them measured around 450 hfe . One was about 260 hfe Link to comment Share on other sites More sharing options...
Leinster Lad Posted November 11, 2020 Share Posted November 11, 2020 They did make a bit of difference. I matched their hfe’s pretty close as possible. I’m going to modify the bf’s heatsink setup so will change them out at that time. See what happens Link to comment Share on other sites More sharing options...
MarcAL Posted November 11, 2020 Author Share Posted November 11, 2020 That is a fair bit of difference. If you can rustle up a matched pair from your stock that may help. You sound like your on the right path. Link to comment Share on other sites More sharing options...
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