Hi,
Wondering if anyone could help,
I am trying to set the Idle current on an aiwa SA-P22, I first set the center voltage to zero and got this spot on.
But I am getting no where near 11mV, in fact mine is reading 25Volts on both sides on TP3, 4, 5 and 6 all reading 25Volts.
The amp is working fine, the relay clicks on and I have no DC voltage on the speaker terminals.
Could there be something wrong with my output transistors or is there a mistake in the manual?
Oh I don't have a DC valve meter, just a cheap voltmeter.
Thanks in advance.
Wondering if anyone could help,
I am trying to set the Idle current on an aiwa SA-P22, I first set the center voltage to zero and got this spot on.
But I am getting no where near 11mV, in fact mine is reading 25Volts on both sides on TP3, 4, 5 and 6 all reading 25Volts.
The amp is working fine, the relay clicks on and I have no DC voltage on the speaker terminals.
Could there be something wrong with my output transistors or is there a mistake in the manual?
Oh I don't have a DC valve meter, just a cheap voltmeter.Thanks in advance.
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Did you play around with different bias levels?
I have owned one of these for a long time and it stopped working recently. I recapped it and replaced a bunch of other parts (especially the relay) and it sounds great now.
There is this website: Adjusting the Bias Of Your Amp [English]
which recommends pushing the bias up to have better sound.
Wondering if I should try it with this little unit.
Hi,
No I left it at 11mV if you want better sound I would recommend replacing the aging electrolytic capacitors especially in the power amp and replace the op amps in the pre amp and tone control , the caps are very close to the heat source (transformer) in a small confined space so will age quicker. Especially replace all the coupling caps anyway. And the filter caps It's not recommended to up the bias too much, as apparently it has very little effect if any on the overall sound, and you might overheat and burn out your output transistors.
The biggest improvement you will get is replace the op amps. My unit sounds so good now, I am testing lots of equipment at the moment during repairs quads 34 pre, 405 / 306 power, leak various model's, armstrong's sugden's, RSC but always prefer my little aiwa. Speakers will be way more important, I have mine driving some monitor audio vintage speakers with caps on cross over replaced with film caps. And a second pair of philips RH 427's but it drives the monitor audio better. Even the cassette deck sounds really nice after a recap and new belts, And it looks really nice.
No I left it at 11mV if you want better sound I would recommend replacing the aging electrolytic capacitors especially in the power amp and replace the op amps in the pre amp and tone control , the caps are very close to the heat source (transformer) in a small confined space so will age quicker. Especially replace all the coupling caps anyway. And the filter caps It's not recommended to up the bias too much, as apparently it has very little effect if any on the overall sound, and you might overheat and burn out your output transistors.
The biggest improvement you will get is replace the op amps. My unit sounds so good now, I am testing lots of equipment at the moment during repairs quads 34 pre, 405 / 306 power, leak various model's, armstrong's sugden's, RSC but always prefer my little aiwa. Speakers will be way more important, I have mine driving some monitor audio vintage speakers with caps on cross over replaced with film caps. And a second pair of philips RH 427's but it drives the monitor audio better. Even the cassette deck sounds really nice after a recap and new belts, And it looks really nice.
My relay burnt out really badly. Those electro-mechanical parts like that don't last forever and get increasingly weaker even if you clean them.
When I had it open, I replaced every electrolytic cap and the awful trim pots with nice sealed pots that have lots of turns. (the ones for the dc offset and bias)
I didn't replace the op amps though. What part did you use for the replacement?
Hi, I replaced the tone control and pre amp one with an OPA2134 (burr brown), mounted on some good quality sockets, so you can try different ones. I found with the OPA2134 there was no need for extra small film caps on the power rails, it made no difference. I tried to replace the phono op amp, but this did not work with OPA2134 as I think the gain was way too high, probably need other modification for it to work. I also left my trim pots alone as I was easily able to set mine, I did put a small drop of switch cleaner, and I figured that the audio signal is not going through these pots, and the bias would have to we a fair bit off for the stereo imaging to sound bad. But will check out the relay next time, they are a great little units.
Not a huge surprise - as evidenced by the voltage divider on the output, they had difficulty getting it stable even with the slower 4558. Someone should have told 'em to include a "Neumann pole" (so the whole contraption doesn't have to go all the way down to unity gain) and maybe 22p between opamp output and -in and/or a 22 ohm - 470p (or so) RC from Q1 collector to ground. Hindsight is 20/20...
While I can recognize why this phono stage design works decently as-is, I think it runs the input transistor hotter than what's needed for MM (Ic ~= 0.7 mA, where 0.25 mA should give plenty low voltage noise already while keeping current noise in check better) and doesn't give it much Vcb at all (literally <0.65 V), allowing Cob to present itself in all its nonlinear glory - if without Miller effect at least.
I think the IC4 (filter amp) position sees a bit too much impedance imbalance (+in: 17k8, -in: 12k1) for OPA2134 to be an ideal choice at full line level input levels. Might try a non-TI NE5532.
For IC5 (tone amp) I'd probably prefer the 5532 again, though OPA2134 should work fine as well.
I see a number of low leakage caps (circled (LL) or (BP.LL)) in the preamp signal path - I wonder how these have held up? They definitely weren't skimping on parts quality there.
A few oddities I noticed on the power amp:
R25/26 (47k) would seem to be dominating input noise. No idea about the input current noise spec on the CX171 (you just don't get any datasheets on very old Sony ICs), but the resistor's thermal noise alone would bring even a perfectly noiseless amplifier up from 33 µV to 114 µV of output noise. Which, mind you, still would be a very good value for normal sensitivity speakers and is likely to be dominated by preamp noise anyway (except when Muting is active on that). 1 pA/sqrt(Hz) of input current noise density would more than double this value though. So I üropose a little test: Activate muting on the preamp - any hiss left?
If C13 (C14) is polarized the right way round, the CX171 would be an npn input device. I would rather expect pnp to be honest. Would suggest measuring the voltage over these caps.
Okay. That makes sense. I'm not using the preamp. the only amp ICs in the power amp are those Sony ones on the LR channel amp boards, and I couldn't find anything that would replace them.
Mine were drifting like crazy, maybe were a lot dirtier than yours. When I replaced them, I was able to dial in the offset and bias very precisely. Maybe not absolutely necessary, but it sure was satisfying.
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