Pioneer SA 408 recap & bias

[nonost]

Hi. I'm dealing with a Pionner SA 408 amp. I want to recap it, bias it and so on. I've attached the schematic.

Almost all electrolytics were Elna caps (dark blue ones, they label says (RE)). The main power filtering were Nichicon and the small ones were also Nichicon.

Here a pic of the main board (preamp & amp section):

And here a pic of the phono preamp (right) and power supply section (not complete):

This is my recap so far:

C53 & C54 Nichicon UPW. These are the two big power filtering ones

C49, C51, C52, C53 & C56 Panasonic FC. All these caps are near the big power supply ones except C51, which is in the phono preamp section.

C1, C2, C11, C12, C46, C47, C31 & C32. All of them were electros. Since they are small values (220nf, 1uf, 2.2uf & 3.3uf), I put film caps there.

In the remain positions I've put Elna Silmic II

For what I know, there's some debate about what is in the signal path and what's not. Since it's not that clear, I made my best based on my little knowledge. But I might be wrong about some positions, please let me know if should do some changes.

There are plenty of ceramic capacitors all around. Should I change some of them for another type of cap? There are lot of small ones (47pf, 68pf, 100pf, etc), but there are also some big ones (47nf). I've never come across any information regarding these, so I'm kind of lost here.

I started a thread a couple years ago when I wanted to recap the phono preamp only. Some very nice users gave me advices about what to do. But I got a bit overwhelmed and realize that I needed to learn more. Some advices were regarding the value of some input/output caps. It turns out that these caps are pretty small in this unit. Is it true? Here the thread I meant:

https://www.diyaudio.com/community/threads/pioneer-sa-408-recap-phono-preamp.339995/#post-5846574

About the biasing:

I've dealt with a few amps of this era and all of them had trimmers to set the idle current and to center the voltage at the output. All of them were class AB. In this amp there are not trimmers whatsoever. There are not big caps at the output as well, so I guess this is a different design. Any ideas about what can I do?

I really appreciate any help. Just trying to learn 🙂

Cheers!

 

[nonost]

And here a couple pics with all the electros removed:

 

[nonost]

There is an Adjusting point mark at resistor R49 (120ohms). Is it that resistor doing some biasing? The schematic says 1.2v at Q6 collector and Q8 base. Is that R49 resistor the bias point?

Cheers!

 

[Mark Tillotson]

I note the opamp for the phono section is a JRC4558 which isn't the quietest choice. If hiss on phono setting is an issue you might want to replace it with a socket and then try a 5532 or a good JFET opamp for comparison.

 

[nonost]

Hi Mark. Yes, my idea is to socket it and try a couple: NE5532 & TL072.

You actually replied to my old thread years ago. And I thought a lot about it. Here the quote:

The 0.22µF after the volume control will affect the response curve as they have a cutoff of 7Hz of so into their 100k loads. This means they are not distortion free, the significant tolerance spread of electrolytics means the channels will not be well balanced at lowest audio frequencies either. 4.7µF electrolytics here will cure both these problems, putting the cutoff sub-Hz. Of course whether such would fit in place of the originals is a question, but film caps would be even bigger and not address the LF frequency response dips.

Should I change them? I don't get why they didn't put the right value. Weren't the people at Pioneer aware of that concept you talked about?

thanks!

 

[nonost]

Yep. That 120r is the idle current adjustment point. The manufacturer says 2mv-110mv across the emitter resistors. That range looks huge to me.