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Old 25th November 2007, 11:56 AM   #1
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Default Jung/PMA Line-Buffer

Hi,
I'm in the process of making a buffer between pot and power amp.

- I shall work with my 100k-log Alps as well with 10k Relais-ladder
- stable
- work effortlessly on reasonable loads
- exhibit a certain fancyness
- sound good

The circuit shows where I am at the moment.
With shown source- and ouptut loads it sims 0.0003% THD.

However, with any other transitor models than shown this number gets worse, so the models might not be accurate.

This is somewhat dependent on the source load, but independent for output loads of 500R and above. Q3/Q4 run at 8mA, Q12/Q13 run at some high 10mA, which helps distortion going down.
I might end with beefier q3/Q4 in the end as well.
The voltage reg. is another area of endless playing...

I might stuff some protection diodes, but this is for later.

Soundwise it has strong and defined bass, and (to my surprise) soft and nice trebles. It plays effortlessly.

My perfboard layout shows some issues with spontaneous oscillations sometimes which need to be solved prior to extensive listening, though.

Rüdiger
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Old 25th November 2007, 05:40 PM   #2
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Default Update

0.000008% THD

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Old 25th November 2007, 07:23 PM   #3
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Default Re: Update

Quote:
Originally posted by Onvinyl
0.000008% THD


Hello Rüdiger, interesting circuit. Supply is around 21V if I am correct? What's the max output before distortion starts to rise appreciably?

Jan Didden
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Old 25th November 2007, 08:37 PM   #4
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Hi Jan,
yes, around 20V. This is to reflect my bench-psu, I'd go for 24V sooner or later.
for 1 Volt AC it's 0.000008%, for 2 Volts it's 0.000018,
3 -- 0.000029
4 -- 0.000042
5 -- 0.000057
6 -- 0.000074
8 -- 0.000118
12 - 0.00024
20 -- 0.000716
25 -- 0.0012
30 -- 0.0022
Above 34, where it's 0.079, it clips.

These numbers make me nervous, somehow to good to be true.

I'm fiddling with a perfboard for this second one, until now I get green fog on my scope, bad oscillations, even the DC points won't follow the simulation.
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Old 25th November 2007, 08:40 PM   #5
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And, to give the credits, this last change is taken from patent 4470020
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Old 26th November 2007, 11:19 PM   #6
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Well, there are issues. It oscillates with low input impedances, and the bad news is, it's the same in reality *and* in the sim. Even the Frequency is close. You can tame this a bit (and half the distortion as well) if you chose 2sc1775/2sa872 as the input pair.
I would have liked to check with jfets there, but I don't get them biased properly.
It shall excell after a high-impedance VAS or anything, but I can't think of a way to have a pot in front of it and keep the extraordinary numbers.

Rüdiger
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Old 26th November 2007, 11:45 PM   #7
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You can use a constant impedance attenuator. They have constant input *and* constant output impedance.

I do not have web references, however if you look up variable Bridged-T attenuators you should find something. They are used in pro-audio and RF work.
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Old 27th November 2007, 12:52 AM   #8
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Quote:
Originally posted by Gordy
You can use a constant impedance attenuator. They have constant input *and* constant output impedance.

I do not have web references, however if you look up variable Bridged-T attenuators you should find something. They are used in pro-audio and RF work.
Here you go:
http://www.ibiblio.org/obp/electricC...ml#xtocid18468

Refer to Bridged-T attenuator near the bottom of the page. Design the impedance at whatever level you desire for your circuit. You can calculate it for 5k, 10k, 20k, etc.

Vary R6 and R7 at the same time (use two gang rotary switch and resistors). The input and output impedance stays constant as the attenuation changes.


You can buy ready-made attenuators here:
http://www.shallco.com/attenuators.html

They say:
" "T" and Bridged-T Circuits
Both "T" and bridged-T attenuators provide accurate adjustments in signal level with zero insertion loss. Increments from 0.1 dB to 2 dB are standard with ranges of 10, 20, 30 and 45 steps in most impedances. In addition to parameters and the devices listed in our online catalog we can provide custom designs for special applications. All devices are manufactured with precision film resistors of ± 1% accuracy and are housed in shielded enclosures. All terminations are solder lug type. Bridged-T type devices are lowest in cost and are equally as accurate as the "T" for all but the most stringent applications. Our applications engineer can recommend the best and most economical device for your application. Standard impedances are 600 ohms but others are readily available. All units are equipped with detents. "


I am sure that they could supply you with something at whatever impedance you need. However it might be more fun and cheaper to build it yourself.
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Old 27th November 2007, 08:49 AM   #9
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Thanks, Gordy,
that was new to me.
It does not work with this ckt, however.
With different T-values, it behaves completley different, from gorgeous numbers (as low as 0.000001% THD) to modulated signal (think AM) up to wild oscillations, all with the same constant impedance but different attenuation values.
It strucks me.
Rüdiger
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Old 27th November 2007, 09:20 AM   #10
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Rüdiger, why don't you settle with a conventional output stage?

My QRV05 works very nicely and it has a design like this
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