My little beast is getting ready..

johnny1 · 13th September 2006, 09:20 AM

Its a douglas Self design.
I beleive you know the schematic but i will try to upload it.

johnny1 · 13th September 2006, 09:31 AM

Its actually two monoblocs with 1KVA trans, and 2x100.000uF/80V (United chemicon)

Here is a pic of what it looks like. Sorry for quality.

One monoblock shown here.
The second one will be ready next week

johnny1 · 13th September 2006, 11:55 AM

This is the Soft start

TomWaits · 13th September 2006, 01:26 PM

Looks a little lite on the Heatsink side of things but looking good.

Shawn.

CBS240 · 13th September 2006, 11:01 PM

Hmm

Somewhere I have seen this circuit but I can't quite place it....I just wonder what would happen to the bias of Q3 if Q6 were to saturate. Perhaps a seperate bias for Q3 and Q6 would make this a non-issue?

Just curious, what would be the difference if you replaced Q8 with a PNP type, use Q5 as the source and Q4 as the mirror, drive the emitter of Q7 with the emitter of Q8(PNP) as a common base VAS instead of a cascode VAS? I have done this and it sounds great, may reduce miller effect but I haven't built a casode VAS to compare. Any thoughts?

johnny1 · 14th September 2006, 10:58 AM

Quote:

Originally posted by CBS240
Hmm

Somewhere I have seen this circuit but I can't quite place it....I just wonder what would happen to the bias of Q3 if Q6 were to saturate. Perhaps a seperate bias for Q3 and Q6 would make this a non-issue?

Q6 never saturates. That's the reason R1 and R12 have different values. The main idea is that Q3 and Q6 should have the same bias.

Quote:

Originally posted by CBS240
Hmm
Just curious, what would be the difference if you replaced Q8 with a PNP type, use Q5 as the source and Q4 as the mirror, drive the emitter of Q7 with the emitter of Q8(PNP) as a common base VAS instead of a cascode VAS? I have done this and it sounds great, may reduce miller effect but I haven't built a casode VAS to compare. Any thoughts?

It would be a different design !

haven't tried that, so i can not comment.
But, you should always take into considaration the absolute phase of the signal (0 deg, or 180deg.) so, you won't end up with possitive feedback.

johnny1 · 14th September 2006, 11:03 AM

Quote:

Originally posted by TomWaits
Looks a little lite on the Heatsink side of things but looking good.

Shawn.

As long as the quiescent current remains below 80mA per transistor it's fine (already tested....).

johnny1 · 14th September 2006, 11:19 AM

and some specs:

145W @8 Ohms (20Hz to 20KHz)
268W @4 Ohms (20Hz to 20KHz)

The transf. has a 2x40.5V secondary, and the voltage on the caps is 54.9V with the amp idling.

At full power the voltage on the caps drops to 53V @8 Ohms and to 52.4V @4 Ohms.
It's the benefits of Overengineering!

Leolabs · 14th September 2006, 02:51 PM

Hi Johnny1!Have you match the output BJTs???

johnny1 · 15th September 2006, 07:30 AM

Quote:

Originally posted by Leolabs
Hi Johnny1!Have you match the output BJTs???

Of course i have them matched!

Not only the output BJT's but also all the complementary pairs.
The differential pair and the current mirror BJT's are also matced pairs.

13th September 2006, 01:26 PM	#4
TomWaits diyAudio Member Join Date: Aug 2005	Douglas Looks a little lite on the Heatsink side of things but looking good. Shawn.

13th September 2006, 11:01 PM	#5
CBS240 diyAudio Member Join Date: Aug 2005 Location: K-town	Hmm Somewhere I have seen this circuit but I can't quite place it....I just wonder what would happen to the bias of Q3 if Q6 were to saturate. Perhaps a seperate bias for Q3 and Q6 would make this a non-issue? Just curious, what would be the difference if you replaced Q8 with a PNP type, use Q5 as the source and Q4 as the mirror, drive the emitter of Q7 with the emitter of Q8(PNP) as a common base VAS instead of a cascode VAS? I have done this and it sounds great, may reduce miller effect but I haven't built a casode VAS to compare. Any thoughts? __________________ All the trouble I've ever been in started out as fun......

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14th September 2006, 02:51 PM	#9
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	Hi Johnny1!Have you match the output BJTs???

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