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Old 22nd December 2012, 04:50 PM   #1021
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Thanks again Nigel for your prompt and extensive replies.

I have finished soldering in one pcb and I hope to test it when I get the transformer. I will be a 300VA toroid with 2 secondaries x 27-0-27V. Do you think that VA rating is enough?

Another point: I remember reading in this thread to check that the resistance across R17 (marked 1K on my pcb, which I used) should be around 800R for minimum bias. In my case I could only get slightly higher than 600R., no matter how far I turned the trimmer pot. Is this reading correct or am I missing something?
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Old 22nd December 2012, 04:53 PM   #1022
Spuds is offline Spuds  South Africa
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Originally Posted by sonata149 View Post
Thanks again Nigel for your prompt and extensive replies.

I have finished soldering in one pcb and I hope to test it when I get the transformer. I will be a 300VA toroid with 2 secondaries x 27-0-27V. Do you think that VA rating is enough?

Another point: I remember reading in this thread to check that the resistance across R17 (marked 1K on my pcb, which I used) should be around 800R for minimum bias. In my case I could only get slightly higher than 600R., no matter how far I turned the trimmer pot. Is this reading correct or am I missing something?
I'm using a 300VA dual 35vac secondary toroid per channel.
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Old 22nd December 2012, 05:07 PM   #1023
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I'm using a 300VA dual 35vac secondary toroid per channel.
Does that mean you have 600VA for a stereo output? Sorry if this sounds like a stupid question but sometimes it doesn't get clear to me, or perhaps I complicate things(?)
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Old 22nd December 2012, 05:11 PM   #1024
Spuds is offline Spuds  South Africa
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Originally Posted by sonata149 View Post
Does that mean you have 600VA for a stereo output? Sorry if this sounds like a stupid question but sometimes it doesn't get clear to me, or perhaps I complicate things(?)
That's correct.
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Old 22nd December 2012, 05:40 PM   #1025
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I thought 400VA was more than enough so I settled on 300VA. But 600VA???
I have sensitive speakers (90db/1W) and rated at 8 ohms. Besides I don't listen to abnormally high volume.

My other question (post #1021) about the resistance across R17, which in my case is 1k, still stands.
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Old 22nd December 2012, 08:12 PM   #1026
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My almost completed amp.

Click the image to open in full size.
Would you mind posting a schematic for the tube stage?
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Old 22nd December 2012, 08:17 PM   #1027
Spuds is offline Spuds  South Africa
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Would you mind posting a schematic for the tube stage?
Standard SRPP using 6922 tubes.

The gain is a bit on the high side, so I'll probably be changing them a bit. Thinking of a shunt or some sort of resistor based attenuation.
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Old 22nd December 2012, 10:55 PM   #1028
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Quote:
Originally Posted by sonata149 View Post
... I remember reading in this thread to check that the resistance across R17 (marked 1K on my pcb, which I used) should be around 800R for minimum bias. In my case I could only get slightly higher than 600R., no matter how far I turned the trimmer pot. Is this reading correct or am I missing something?
The highest resistance indicates that you have the bias current wound down to min. and that's all you really need to achieve. The resistance may vary from one clone to another so don't be over-concerned that you can't achieve "around" 800R. The purpose is to ensure the pot is set in the minimum bias current position.

The important figure will be the actual bias current you then set when the amplifier is powered.
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Old 23rd December 2012, 01:24 PM   #1029
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Originally Posted by sonata149 View Post
I thought 400VA was more than enough so I settled on 300VA. But 600VA???
I have sensitive speakers (90db/1W) and rated at 8 ohms. Besides I don't listen to abnormally high volume. .

The rule I use is the VA should be twice the total output power . Be very careful the off load voltage does not exceeded the maximum ratings of the amplifier or capacitors . Allow at least 10% for safety .

In theory a transformer 1/4 this size will just about work . Some low grade designs are built this way . To be clear 2 x 50W requires 2 x 100 VA minimum if running an industrial application ( a motor drive ) in conventional class AB as Naim . Some say the most punishing music is Techno at 1/3 industrial rating . That implies 66 VA would be enough . In practice even the most cost obsessive company would not go that low . I seem to remember the Quad 405 was 300 VA for 2 x 100 W , that was perhaps as far as anyone should go in cost cutting a high quality design . That is 50 % above total output power . Something to remember . Many amps sound OK when giving as much as 10 % distrotion . Sometimes the power output is considerably higher than book specification . That is where the Quad is perhaps a bit small .

Class D for TV's seems to be a different world , very mean . High book power ratings which in practice are never used . If building class D I would still use the minimum recommendations for class AB ( + 50% ) , too much of the spec sheets I suspect is TV based . Transformers are cheap and like having a big engine guarantees results . Too big is perhaps 1000 VA , it brings fuse blowing problems . Even 400VA if the transformer is not the best .

Class A is usually well described in any design . It requires more VA .
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Old 23rd December 2012, 07:17 PM   #1030
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Thanks folks for your insightful replies. Regarding the resistance across R17, I reasoned that the 200R difference is due to the difference in the value of R17 - 1K as against 1.2K.

Regarding power transformer VA rating for Class AB, that's a good rule of thumb to keep in mind: 2x the total power output.

Thanks to all who contributed to this site.
Best wishes for this festive period.
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