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Old 7th February 2014, 02:17 PM   #3921
AndrewT is offline AndrewT  Scotland
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The B1 is essentially a SE ended ClassA amplifier with a gain of just a tiny fraction less than 1. I have found that a cascade of 5 of these B1 loses about 4% of the signal level. That makes the gain of a B1 ~ 0.99 (-0.1dB)

A single ended ClassA amplifier has a maximum output current in the opposite direction of the supply polarity of just a fraction less than the bias current.

If you use an 8mA Idss device and bias it at 8mA, then the maximum output current is ~-7.9mApk to your load.

If you use an 4mA Idss device and bias it at 4mA, then the maximum output current is ~-3.9mApk to your load.
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Old 26th February 2014, 06:38 AM   #3922
exwiso is offline exwiso  Australia
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OK I'm completely new to all this and have never attempted anything like it in 60+ years. I've looked this project a number of times but every time 'till now courage and PCB stock have been in inverse supply. Now at last I have Mr Pass's board to hand, and have Mundorf caps and Audio Note resistors on their way.

But the attenuator value is driving me nuts.

I have trawled though all the hundreds of pages of this thread and can’t see any guide, other than to try a few different values and see what works. I was hoping to go with something like the Goldpoint mini-v so I would really like to get it close first up

My valve mono blocks have an input impedance of 100K Ohms

The Slee Reflex M phono (+Grado Ref Sonata) gives “Output impedance (driving impedance): 1k Ohm (will drive 10k Ohms and above)”

I can’t find the output for the Cayin 17A CDP (output 2.3v?) but a direct connection between player and power blocks would blow out the windows. I normally run the cdp at about half the volume phono. I use chord anthem interconnects (1 metre).

My inclination is to the 20K (Special) Goldpoint Mini-v but only because it is at a pretty good price at the moment and my completely ignorant guess is that I should pitch lower rather than higher.

There is also quality in the one from JAC Music in Germany very well priced but it seems to be only in 47K. Is that too much?

Any help, suggestions or recommendations gratefully received.
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Old 26th February 2014, 07:26 AM   #3923
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22K is the recommended maximum from Salas for the DCB1 I'm going to use a 25K pot, as I thought that was the upper limit I had seen posted at some point. 20K sounds like a good choice

I'm currently doing a verro board DCB1. Hopefully won't be too long before it is finished!

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Old 27th February 2014, 03:41 AM   #3924
exwiso is offline exwiso  Australia
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Thanks.

25k was the value used by Mr Pass in the original diagram and commentary. I became a little concerned when my 100K power amp input seemed to be at least twice that of other examples given. Where I have been able to find inputs for retail Pass amps they seem to give an input of 30/20K so was this in the thinking of the creator when he chose 25K? That's roughly 1:1; so should I be looking at 100K?

I thought I saw something somewhere about a 1:10 ratio which is why I was thinking 10-20K.

But I could have dreamt that. It's been on my mind a lot lately.

David
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Old 27th February 2014, 04:41 AM   #3925
omishra is offline omishra  India
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Output of Pass B1 can drive any power amp (IMO). You need not worry driving power amp of input impedance 100KΩ.
You can connect most sources to Pass B1 buffer while you use volume control pot of 25KΩ above.
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Old 27th February 2014, 05:52 AM   #3926
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Quote:
Originally Posted by AndrewT View Post
The B1 is essentially a SE ended ClassA amplifier with a gain of just a tiny fraction less than 1. I have found that a cascade of 5 of these B1 loses about 4% of the signal level. That makes the gain of a B1 ~ 0.99 (-0.1dB)

A single ended ClassA amplifier has a maximum output current in the opposite direction of the supply polarity of just a fraction less than the bias current.

If you use an 8mA Idss device and bias it at 8mA, then the maximum output current is ~-7.9mApk to your load.

If you use an 4mA Idss device and bias it at 4mA, then the maximum output current is ~-3.9mApk to your load.
But only if distortion does not matter.
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
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Old 27th February 2014, 06:05 AM   #3927
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Quote:
Originally Posted by spiri View Post
But only if distortion does not matter.
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
Monotonically so in B1's case. Expect 0.003% at 2V RMS on test gear HighZ load.
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Old 27th February 2014, 06:25 AM   #3928
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HighZ load > 20k ?
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Old 27th February 2014, 08:58 AM   #3929
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by spiri View Post
But only if distortion does not matter.
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
Of course distortion matters.
All SE amplifiers (without global NFB) distort if driven to near clipping.
That's why I strive for the Source to have a clipping voltage at least 10dB above the maximum input signal to the power amplifier.
A B1 running on 18Vdc has a maximum unclipped voltage of 6Vac.
A power amplifier with a maximum power sensitivity of 1.5Vac will leave the B1 with an unused 12dB of overhead.
i.e at absolute maximum output from the power amplifier the B1 is passing a 1.5Vac signal.
The current passing the B1 is a completely different story. It depends on the impedance of the cables and load/s connected to the B1.

If the Idss of the B1 jFets were 8mA and the bias of the jFets were 7.9mA (since the voltage is 9Vds, not 10Vds) then the maximum unclipped output current capability will be around 7.8mA.

1.5Vac (2121mVpk) into a 20k load resistance is 0.1mApk output current into the non reactive element of the load.
Allow 1mA of peak current for the capacitive elements of the load gives an absolute maximum output current of 1.1mA.
That is 14% of the B1 current capability.

Music does NOT stay at absolute maximum peak transient power all the time.
Replayed at -20dB relative maximum power and the voltages (and currents) will be one tenth of the maximum values.

That takes the output current from the B1 down to 1% of capability as an average for normal music listening.

Now that I have explained the B1 operational currents and how they relate to bias current what does your comment mean?
Quote:
A peak signal current about 10% off the IDSS is acceptable, then the distortion increases.
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Last edited by AndrewT; 27th February 2014 at 09:02 AM.
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Old 27th February 2014, 10:11 AM   #3930
Salas is online now Salas  Greece
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Quote:
Originally Posted by spiri View Post
HighZ load > 20k ?
Yes, not the 50 Ohm lab standard or the 600 Ohm pro audio standard.
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