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Old 18th April 2002, 09:31 PM   #81
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Quote:
Originally posted by Geoff
(...)
The ccs has reasonable supply rail rejection and there has been no problems with hum/noise so far, though the amps using it have had regulated supplies. I would expect more breakthrough from the negative rail than through the ccs. If you are concerned, the supply rail rejection of the ccs can be improved by splitting R3 ('JLH for ESL' schematic) into two and decoupling the mid-point to the +ve rail with a 47uF capacitor.
I thought of something similar too... actually, my supply is a fairly nice working cap-x, so hum is not much of an issue, but it always surprised me a bit that Mr Hood decided to place a regulator there when there're better (IMHO) options, like the discrete ccs.
I'll try the mod and get back to you when this amp is (finally) complete. Thanks again for the feedback!
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Old 18th April 2002, 09:51 PM   #82
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Default Lisandro_P

You could create a negative reference of two forward baised diodes and put a trim pot across them. Connect the 47K input resistor to the wiper of this pot and you can pull the base down so that emmiter is sitting just below ground. The pot can be nulled for zero offset at output. You really might want to ac couple the feedback resistor to ground since the DC offset is going to drift on this thing. Especially without the benifit of a long tailed pair diff amp for a front end.

H.H.
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Old 18th April 2002, 10:22 PM   #83
Geoff is offline Geoff  United Kingdom
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Harry's suggestion will work and was in fact used on yet another version of the JLH Class-A, the Class-A headphone amp published in Hi-Fi News Jan/Feb 1979. If this method is used, the polarity of the feedback capacitor to ground will need to be reversed since the emitter of Tr4 will sit at -0.65V as opposed to +0.65V for the 7815/ccs method. A 100uF capacitor across the diodes will also be beneficial.

The dc offset variation with supply rail voltage and/or temperature will be greater with the diode/resistor arrangement than it is with the 7815/ccs. I quote from another JLH article for a 270W/4ohm amp that uses a similar input stage and NO capacitor in the feedback leg to ground:

" In order to minimise the effect of common mode distortion at the input, while preserving the facility of a direct coupled structure, the customary ' long- tailed pair' has been replaced by a single transistor, in which the offset which would otherwise arise due to its base-emitter voltage and the emitter circuit current flow through the feedback resistor, is removed by an additional current source employed to inject a sufficient quantity of current into the emitter of the input transistor to offset this voltage. Again, since the input transistor remains at ambient temperature, the constant current source can be arranged to track this thermally, and remove errors in dc offset due to ambient temperature changes."

Geoff
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Old 18th April 2002, 11:40 PM   #84
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Red face Base bias

Since I did a bit of a poor job explaining...... In reference to the schematic below. The quiesent current of TR4 is about 1.3 volts divided by 8.2K ohms or 0.16mA. With a cap (to ground) connected to R6 and no current source, the only DC path is though R8. The output is sitting at 0 volts, so the emmiter should be at about -0.16mA multiplied by 2.7K or -0.43 volts. This neglects the base current of Tr4 through the 47K resistor which is small and could be nulled out. The bias for the -1.3 volt reference could be supplied by a JFET current source for good noise rejection and stability with changing supply voltages. R2 could be connected to the wiper of a 10K ohm trim pot across the negative 1.3 volt reference. An LED could be used for a reference also since the base voltage is being adjusted the trim pot. The exact reference voltage is not critical as long as it is stable and low noise. A green LED would give about -1.85 volts. I think the temperature stability should be fine.

H.H.
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Old 19th April 2002, 12:19 AM   #85
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Default Base Bias

Revised schematic:
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Old 19th April 2002, 02:02 AM   #86
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As I said in my last post, this has been done before. See Circuit 1 at:

http://www.gmweb.btinternet.co.uk/jlhphones.htm

As Harry has suggested, R3 in Circuit 1 can usefully be replaced with a Jfet ccs or similar and a pot can be incorporated to provide a means of adjustment.

Geoff
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Old 19th April 2002, 06:53 AM   #87
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I like the ccs solution better. Thanks!
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Old 19th April 2002, 07:37 AM   #88
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Lightbulb again a bit more in depth with this schematic

I discussed the schematic with a friend and he suggested that R9 is a source of some of the distortion. There is a current flowing from Tr4 to Tr3 to Tr1 which changes with audio signal, but it's not linear and because of that there forms a small voltage on R9 that adds to the output signal and also is not linear. As far as I understand it should be possible to leave this resitor out.... Do you guys see any possible problems with that and consider it worth exploring?

**

Geoff, do you know if anyone has tried to use some other method than Tr5 current source in JLH'96 or the original resitor current source? This is the point that troubles my mind the most and I would like to think through any possible solution and then try the few most appealing.

Ergo
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Old 19th April 2002, 07:43 AM   #89
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scottnixon, could you please mail the Bedini circuit to me too. It interests me, because I have one project of my own design that has the same front end but I ran in trouble with output and stability. This I quess would give ideas how to do it a bit different

My mail is ergo.esken@mail.ee

Ergo
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Old 19th April 2002, 09:42 AM   #90
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I have asked Geoff about the R9 also some time ago... I did that because Rod Elliot made a observation that a high R9 has more distorion but only second harmonic, and low R9 has lower distortion but mainly third harmonic...
My simulator show give any differnce


so .......????? I don't get it...



greetings,
thijs
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