Output transistors subs with higher gain

[david winter]

Hi,

I've been wondering whether the gain of output transistors is critical if the subs have 2 to almost 3 times the gain of the originals and what changes should be done to compensate.
I'm interested in this quesiton in a general case after trying to find subs for a Sony output stage using 2SD291 (gain around 60) which schematic is below. Thanks.

 

[dr*audio]

More gain is better. It makes the transistors easier to drive from the driver transistors. No modifications required.

 

[david winter]

Thanks. That was fast !
Are there some cases where components around them should be changed ? Or even a maximum gain for the subs ?

 

[gpounce32768]

If a sub makes the volume increase too rapidly as the knob is turned (eg goes full volume with the knob at 1/2 or less), you can increase the negative feedback (assuming this is a feedback design), generally done by decreasing the feedback resistance. Some care should be taken to decrease it properly, some designs a single resistor, others may need a couple to be changed to keep things balanced.

 

[david winter]

Here the feedback is a R + R//C : 6.8K + (3.9K // 0.47µF).
I'll check the volume. Maybe the 6.8K can be replaced by a 10K trimer to adjust this.

 

[gpounce32768]

Adjusting series resistance of the volume into the amp stage works too, though I've found that may need a considerable increase to make a difference (eg several 100% change) which I think may risk attenuating low volume components of the signal into the noise. wrt the feedback mod, selection and adjustment of the resistor portions depends on topology as well. I found cut-and-try with resistors on the feedback circuits of the 2 amps I've done it to worked- 25% reduction each try sort of thing.

 

[gslikker]

I think too much gain could be critical if there is NO resistor from base to emitter of such outputs to have at least some driver current NOT going into the base of the outputs.
In your case you have R554 and R555 taking care of that.

(The gain of the output transistors has nothing to do with amount of volume)

 

[dr*audio]

Right, in the outputs its current gain, not voltage gain.

 

[david winter]

Ok I have anothe issue now but not complicated at all.
The outputs are there, I changed the trimmers and one channel works but not the other due to a shorted 2SC1364 (the schematic is the older version; the board is originally fitted with a 2SC1362 followed by 3 x 2SC1364).
From what I read on AK, which of the following subs would best replace them ?
- KSC1815 ? KSC1845 ? I have plenty of both as you might guess !
- KSC2383 (same)
- KSC945 (don't think I have some except pulls)

How about a KSC1815 first, followed by KSC1845 ?


Thanks !

 

[glen65]

Hi,

I've been wondering whether the gain of output transistors is critical if the subs have 2 to almost 3 times the gain of the originals and what changes should be done to compensate.
I'm interested in this quesiton in a general case after trying to find subs for a Sony output stage using 2SD291 (gain around 60) which schematic is below. Thanks.

2N3054 should work, very few out there who actually still manufacture the TO-66 case.
https://www.mouser.com/ProductDetai...=sGAEpiMZZMshyDBzk1/Wiw99kSkYzPxmnk5Hx0Lh8Pk=

 

[david winter]

For the outputs I used 2SD1135. Different package but mount very easily with no pinout problem.
For the 2SC1364 I used KSC2383, they work fine.
Now the amp works well and remains at 15mA on both parts.
Just in case some others have a Sony HP-211A to fix and need subs that aren't hard and expensive to get...
Thanks for all who gave good advises, that was much appreciated.