Picking up SS williamson amp - is it worth moding?

[DJNUBZ]

I am picking up a pair of Twin-20 amplifiers from this website. You can find some into on it here.

Hi-Fi, DIY Projects

Here is the amplifier circuit

http://www.saturn-sound.com/images%2...20amp'.jpg

Other then upgrading caps do you guys see anything I could do to improve this circuit?

[lineup]

Looks like a nice amplifier.
Has got bootstrap capacitor at the innput.
The voltage supply is high: 68V
So, it is not a Class A amp.

Do you have any parts list?
THe transistor types does not show.
Maybe it can do with any modern transistors.
Well, here is a parts list: http://www.saturn-sound.com/images%2...20part%202.jpg
Should work with 2N3055 as output.

[lineup]

It is certainly worth modifications!

See diagram.
I have done mods:
- changed values of C6, C4 and C8 because it was overcompensated
- using diodes 1N4004 they are perfect replacement in bias setting
- using transistors 2SA970, BD139 and BD140

Result:
Only 0.004% THD at 1 Watt
This is very good. Not far from being called Hifi.
I run the 2N3055A at ~50mA bias.
The output hit the 50% of 68V right on the spot, without any adjustment!
Max power with reasonable distortion seems to be 50 Watt.
THD 0.057% at 50 Watt out into 8 Ohm

[DJNUBZ]

That is awesome, thanks. Once I get the amps I am going to listen to them for some time then try upgrades so I can learn how they affect sound.

If I were to remove the input cap would I start causing problems? Are there any other caps I can just remove and not run into issues?

Since you have it modeled could you tell me how it performs with original parts into a 4 ohm load? I don't know how well their measurements at 15 ohms correlate.

[lineup]

It is NOT possible to remove input cap.
It has to be there, as this is a single supply.
If it was +/-34V dual supply we could remove input cap.
Now, input cap can be of such good quality they disturb nothing.
Which means there is nothing to gain to remove them.
But they are there for a purpose, so we leave them in place.

I should not remove any caps in this amp.
But we can, like I did, get some benefits by changing values of caps.

I have not models for the 4 smaller transistors.
But I could run with original values on capacitors and with as many original parts I have.
And with 4 Ohm load.
I think it will do pretty well in 4 Ohm, too.
I be back.

[lineup]

It delivers 86 Watt into 4 Ohm at THD 0.2%
It delivers 74 Watt into 4 Ohm at THD 0.1%
This test is done with original values, as close as to the original amp.
Results are slightly worse than my previous posted version.

1kHz into 4 Ohm resistor
01 Watt 0.011% THD
10 Watt 0.020%
20 Watt 0.030
30 Watt 0.035
40 Watt 0.040
50 Watt 0.049
60 Watt 0.062
74 Watt 0.100%
86 Watt 0.200% THD

[DJNUBZ]

Thanks for all the help, I really appreciate it. I can't wait to get these.

I noticed you left the 2n3055 output transistors in yet replaced all the other transistors. Is there a reason for leaving them? Is there a better/newer alternative?

What program is that you are using to model this circuit?

[john_ellis]

Hi DJNUBZ
I'd suggest a couple more changes. You could keep a quasi-output stage or consider using 2N3055/MJ2955 complementary. Either way I would also make another couple of changes:
- replace the bias diodes with a transistor on the same heatsink as the output devices
- remove the 470 ohm base resistors in the drivers
- for protection, add a small resistor in series with the VAS emitter, add a couple of diodes in series between the base and ground, but keep the 680 ohm base resistor between the base and emitter.
Distortion at higher frequencies might be reduced if you changed the Miller capacitor and compensation a little...but it may be a little more finicky to stabilise.
John

[DJNUBZ]

So I got the amps in and took an initial listen. For a 40 year old design it's not bad but it doesn't keep up with my zen select amp, yet. One of the amps works great. One of the amps blows a fuse when I give it power so I have to pull it apart and see what is going on.

John I'm an amp newbie so some of what you are suggesting is a bit beyond me. I am going to just start with removing and replacing parts.

Lineup it seems that some of the caps you have changed have also changed designations. For example it looks like C1 on the schematic I have is C5 on yours. Is there a reason for this or is it just the way the program labeled them?

Once I get the second amp working I think I am going to mod one and leave the other. My first plan is to just replace the D1 and D2. My next step will be to upgrade the transistors. My last step will be to try those other cap recommendations. I am also thinking about removing any protection from the PS and just using fuses to protect everything. Is this a bad move?

[ilimzn]

Lineup, I think the bias circuit and diode in series with the bottom putput CFP need some attention (convert straight CFP to baxandall diode setup, there are 3 - or 4 if baxandall diode setup used - Vbe drops in the output stage but only two in the bias circuit, investigate possible thermal instability?)... also, something like a good pair of cheap 2N3773 instead of 2N3055A might be a nice improvement.