Pioneer SX-1250 Repair Manual

[steveUK]

With bipolar outputs, if you observe on a distortion analyzer, you adjust the bias until the distortion falls away, usually around 10mv. The idea is to switch on both comp pairs so the switching dist. goes away.

Some feel there is no benefit to more quiescent current if both transistors are on.

Douglas Self writes that a range of about 43mv-52mv is enough for the commonly used emitter resistors, i.e. 22,33,47, 1ohm.

But there is obviously differing opinions amongst amp designers.

Hope this helps.

Thanks for the info. Well, 10mV or 43mV-52mV it is then. Or, if one decides sticks to the Pioneer spec, 100mV.

Question, if one is using the receiver in a situation where not much volume/power is being used, and the ambient temperature is 'not high', then setting a higher quiescent current (up to 100mV) gives less distortion then, that right? Notwithstanding the fact that "Some feel there is no benefit to more quiescent current if both transistors are switched on." I'm thinking that Pioneer, amidst the receiver wars that raged at the time may have used a higher value to reduce the distortion figure - even just a tad - to gain spec points in the war? Could that be the crux of the matter? Even though they knew that in doing so the receiver would run on the hot side?

 

[tarior]

Setting quiescent current higher reduces distortion, to a point, and then no more gains are made. So, there is an optimal setting. Apparently the engineers at Pioneer decided that this is the optimal idle current for this amplifier.
Though it does seem a little high.

 

[gslikker]

It could be set depending on worst-case design (read: even if all current driver transistors are at low side of spec, amp must work okay).

Now, higher temperature will also get you higher HFE (which could be important). There are 4.7 ohm resistors in the base lead of the output transistors, guess it does not make the amp any faster....

There are other discussions on other high power amps where even higher bias is used for ultimate speed.

I think it is all a compromise.

I wonder if the newer transistors, many times having a quite high HFE graph from low currents up to 6 amps or more, could reduce bias needed.

 

[t j]

With bipolar outputs, if you observe on a distortion analyzer, you adjust the bias until the distortion falls away, usually around 10mv. The idea is to switch on both comp pairs so the switching dist. goes away.

Some feel there is no benefit to more quiescent current if both transistors are on.

Douglas Self writes that a range of about 43mv-52mv is enough for the commonly used emitter resistors, i.e. 22,33,47, 1ohm.

But there is obviously differing opinions amongst amp designers.

Hope this helps.

I wonder why the manual say 100mV and how many units have been fried because of this error?

 

[steveUK]

I wonder why the manual say 100mV and how many units have been fried because of this error?

Well, it hasn't been identified as an "error" as such, more a case of 'it seems high, but that's what Pioneer wanted'. Assuming 100mV is not a typo, I doubt if any units have been fried as a result of this alone. If that was the case, with the number of threads and posts about the SX-1250 over the years, it would have been 'discovered' yonks ago and a well documented fix prescribed.

 

[tarior]

It could be set depending on worst-case design (read: even if all current driver transistors are at low side of spec, amp must work okay).

Now, higher temperature will also get you higher HFE (which could be important). There are 4.7 ohm resistors in the base lead of the output transistors, guess it does not make the amp any faster....

There are other discussions on other high power amps where even higher bias is used for ultimate speed.

I think it is all a compromise.

I wonder if the newer transistors, many times having a quite high HFE graph from low currents up to 6 amps or more, could reduce bias needed.

According to ON, if one were to gain match MJ21195/21196, THD could be reduced by a factor of 10. Doing this could likely give the result of needing a much lower bias value.
Not sure if it's worth buying and sorting through a big pile of transistors for that.
http://www.onsemi.com/pub/Collateral/MJ21195-D.PDF

 

[gslikker]

At work there is some guy who worked in some audio service center long ago, he stated Pioneer always sent matched transistor pairs, so you can assume originals were matched, also.

I wonder about what circuit they (ON) will really use, instead of the "incomplete example"of the one in the datasheet you provided.

 

[tarior]

I wonder about what circuit they (ON) will really use, instead of the "incomplete example"of the one in the datasheet you provided.

Your guess is as good as mine. In electronics school, I remember setting up test circuits for measuring hfe. Nowadays, I think that I would just use my Duoyi, or a Peak Atlas, or something like that.

 

[drumbum]

I was looking at the manuals of some of the pioneers "around" the 1250 and the 1280 specs 15mv. Most below the 1250 speced similar low idle current.

I think the idea of high idle current is to try keep an AB design amp to stay in A longer before switching.
Another "theory" is to get the unit hot as quick as possible.

Went these rounds recently setting bias on a B&K amp that i repaired, and the "specs" didn't make a lot of sense. Settled on 40mv, plus mods, like a fan and discrete ch. switches.

Just a novice sharing thoughts.

 

[steveUK]

I think the idea of high idle current is to try keep an AB design amp to stay in A longer before switching.
Another "theory" is to get the unit hot as quick as possible.

What is the advantage of that?

 

[steveUK]

At work there is some guy who worked in some audio service center long ago, he stated Pioneer always sent matched transistor pairs, so you can assume originals were matched, also.

I wonder how that sits with the 'change everything because it's old' brigade? I don't recall it being deemed a necessity to perform swaps with matched pairs, so effectively, assuming the 'old' trannies are still working, it's actually a bad swap.

 

[drumbum]

According to ON, if one were to gain match MJ21195/21196, THD could be reduced by a factor of 10. Doing this could likely give the result of needing a much lower bias value.
Not sure if it's worth buying and sorting through a big pile of transistors for that.
http://www.onsemi.com/pub/Collateral/MJ21195-D.PDF

Recently went through a QSC dual mono 375w amp. One channel factory w/toshiba 554-424 and the other repaired w/10 each 21193/94. Toshibas needed 50mv for the distortion to fall off and the On-semi's needed only 20mv.

 

[gslikker]

I wonder how that sits with the 'change everything because it's old' brigade? I don't recall it being deemed a necessity to perform swaps with matched pairs, so effectively, assuming the 'old' trannies are still working, it's actually a bad swap.

I have no idea, I have never worked at an audio repair facility. I presume they repaired according quality guidelines using original components, hopefully.

 

[gslikker]

.....xcuse, double post....

 

[calman46]

I just finished doing the AK SX-1250 restore. It's my second one. I have the MJ21193/94 outputs. I set the bias for 91mV first, and it was drawing 1amp and the sinks got warm quick I thought. I lower the bias down to 65mV its drawing just under an amp, but it seems to be running cooler. I have to check the other SX-1250, I don't remember what I set it at. Anyone care to comment why you can run the Onsemi outputs at the lower 60mV's? Thanks