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Old 27th March 2013, 08:56 PM   #2341
oiphy is offline oiphy  Norway
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R19/R29 (input to ground) = 10k - this seems to be a decade higher than input resistor in most other designs... Why is it 10k here?

it's actually 22.1KOhm

Quote:
Originally Posted by Eric View Post
I forgot to include in above, when I moved to Graeme's CSS for the input differential, I had to change VR2 from a 200R to a 1K pot in order to adjust absolute offset to 0v. The 200R pot didn't give me enough room to bring offset down to zero.

Using Graeme's schematic for labels, I have the following:

R18/R28 (only resistor between input signal and jfets) = 10k
R19/R29 (input to ground) = 10k - this seems to be a decade higher than input resistor in most other designs... Why is it 10k here?
R16/R30 (feedback) = 100k
R47/R47 (McMillan) = 10k
R23/R25 (connects ZTX550 collector to -Rail) = 392R
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Old 27th March 2013, 09:34 PM   #2342
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include some 10-22R source resistors for input Jfets

that will influence overall tempco
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Old 28th March 2013, 07:54 AM   #2343
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Quote:
Originally Posted by Eric View Post
I forgot to include in above, when I moved to Graeme's CSS for the input differential, I had to change VR2 from a 200R to a 1K pot in order to adjust absolute offset to 0v. The 200R pot didn't give me enough room to bring offset down to zero.
Hi Eric,

I'm afraid something is wrong here. The absolute offset is set by the voltage over the drain resistors (392R), This is defined by the bias through the input diff pair. The bias is defined by the current source.

Since you didn't change the drain resistors, the bias should stay the same so the current source setting also should stay the same. If it is not your input arrangement must have influenced the current source. Are you sure there's enough voltage drop over it to work properly?

William
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Old 28th March 2013, 01:04 PM   #2344
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William: I didn't check this. I presume the proper place to measure voltage drop is between the source pin of Q6 and the positive rail?

In the Aleph-J circuit, Nelson specifies this as 8.3v, but in the Aleph 2 and 4 amps, it is specified as 4-5v with a 9v drop across the zener. Should these measures in my JFET amps match my amps with the 9610 front end?

Zen: Thanks for the suggestion. I presume you mean one R for each JFET? This means total disassembly again. I did notice a few things when playing around with relative offset. If I put my finger on top of the 550s one the "Q5 side" of the circuit, the offset goes positive quickly. If I put my finger on top of the 550s on the "Q7 side" of the circuit, the offset goes negative quickly. If I pinch all 4 in a row together with my fingers, offset approaches zero, so I'm wondering if the outer two 550's in my lineup of 4 are dominating the relative offset swing. I was thinking of trying another arrangement with better thermal coupling of the 550's.

I do notice that when rel offset is low (~14mV on powerup) the hum/hiss (tweeter and midrange drivers) from the speaker is very, very quiet. After everything warms up (and offset is now ~80mv), there is more pronounced hum/hiss from the speaker that matches my other amps. Don't know if this is linked to overall thermal state, or just relative offset (or if these two are actually more identical that I'm thinking).
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Old 28th March 2013, 03:02 PM   #2345
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Hi Eric,

yes. This voltage is defined by the gate voltage and Vgs of the 9610. These should have stayed the same but didn't. If you had to raise VR2 to get it right then less current is coming from the current source.

Since absolute offset is zero the current at the drain resistors must be the same as before. So where is the extra current coming from?

I had a look at the schematic and the only thing that could be is that the base current of the ZTX550's is added to the bias current. I would have thought this to be very small but maybe it isn't. MAybe the data sheet will clear this.

William
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Old 28th March 2013, 05:08 PM   #2346
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no way - for ZTX base current ;it's too small and it will influence current through 390R only in case that ZTX is ooked , when that isn't base current anymore , but ooked current

fact is that output mosfets are biased , so current through 390R resistors is adequate
sole question is how Eric came to that current ?

however , measuring voltage across zenner (+1n400x ?) or - written in other way - from gate of CCS mosfet to positive PSU leg ......... and exact voltage between mosfet source and positive PSU ....... and exact resistance between mosfet source and positive PSU .......... will tell us more about proper operation of CCS
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Old 29th March 2013, 07:10 AM   #2347
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yes, that's what I thought.

does it sound better with ooked current?

William
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Old 5th April 2013, 07:04 PM   #2348
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I've been a bit tied up with work and weekend travel lately, so my progress has been a bit stalled. Thanks to Zen and William for suggesting measurements on the CCS - I will measure this soon and post the results.

I've been thinking about the tempco of the new front end - particularly the impact of the 550s on the relative offset drift that I have as the amp warms up and I touch them. It then occurred to me that these are the ONLY parts in my amp that have not been matched across sides. Duh...

So, how do I measure the ZTX550s to attempt to better match them? Can I just do this the same way I did with my 2SJ74's?

Perhaps this will solve my wandering offset difficulties. I've been trying to come up with a simple/elegant arrangement to thermally couple them, but this just results in messy wiring.
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Old 5th April 2013, 07:51 PM   #2349
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no need for that , actually

they can influence Vds of Jfets just a little , and that difference can't influence Jfet's current that much

however - matching them just on Hfe function of your DVM is sufficient

everything is going in circle LTP CCS tempco-Jfet tempco-output mosfets tempco ..... that middle one being most influental
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Old 20th May 2013, 03:51 AM   #2350
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Default how much volt do aleph-x ......?

how much volt do aleph -x need at it's differential input to drive it at full power?
i need to choose an usb dac but trying to find one can drive the aleph-x directly so.......
my aleph-x has 24 irf240 driving at 30 volt + and - 8 amp current
please helps
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