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Old 8th October 2006, 10:58 AM   #121
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Hi Rob

Yesterday evening, I increased feedback resistors from 56K to 100K,
Rin still being 15K, and Rout 1K. The offset stabilty was not affected
by this change (0.1mV stable on both channels when I went to bed...).
But it had heatsinks on, so I can't tell without heatsinking.

On the scope, absolutely no overshoot at 20kHz (the maximum clean
square signals frequency on my generator) with my pcb layout.
I didn't have 150K resistors, so I couldn't test.
Clipping begins to occur around 4V peak to peak at the input (unbalanced).

This morning, I made some listening, and honestly, apart from
a greater gain, so louder sound , I couldn't find a difference,
but the listening session was somewhat short, so I'll go on this evening.

But as I don't already have two sets of modules to compare, I can
only rely on my memory, and it's not the most impartial judge

Cheers
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Old 8th October 2006, 02:18 PM   #122
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Hi Cheff

Thatís surprising I need to put 10pf parallel to the feedback resistors to avoid overshoot.
Maybe the overshoot has something to do with the BC550/BC560 ?
Which software do you use for simulating.
Tomorrow I listen again to the 1K to ground and compare it.
The best way to try such things is to swap modules.
Maybe mine sounds better now after some burn in time

Rob
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Old 8th October 2006, 03:33 PM   #123
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Hi Rob,

WRT overshoot, I don't know if the BC550/560 are responsible...
I'd blame it on the perfboard layout, since I found it to be
very layout dependent. Using zetex all along, I've had boards with
overshoot, while other hadn't...

For simulations, I use Microcap (www.spectrum-soft.com)

I agree the best way to compare should be to swap modules, but
unfortunately, I can't for the moment.

And for burn in, I'm pretty close to agree with N.P. :
http://www.diyaudio.com/forums/showt...567#post214567



Going back to listen

Cheers
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Old 8th October 2006, 07:12 PM   #124
SDF is offline SDF  France
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Hi all!

Is there anybody knows the output impedance of this schematic?

I look this schematic for a preamp but I need gain of 6! If I remove the Rin ang directly plug a potentiometer there is not any problems doesn't it ? (Imade a pic )

My first idea was to plug attenuator, like Flat , in output of the UGS circuit but for my application, the output impedance will increase too much!

See you
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Old 9th October 2006, 03:25 PM   #125
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Hi Cheff

Today I did some measurements and the overshoot is due the 10k on the output not the feedback resistors.
If I return to 1k I donít have any overshoot.
So maybe the sound difference is from the output resistor to ground and not the feedback resistors or both.
Another problem is that you have overshoot if you connect a cable or put some capacitane to the output.
The overshoot happens from about 100pf to 10n.
That are typical cable capacitane properties.
I donít have any resistor at the output so maybe you donít have that problem.
Now I find out what I need for resistor value in series with output or place an RC to ground.
Further I have to wait till my boards arrive for my relay attenuator.
And place it on the output see if there are any difference in sound.

Rob
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Old 10th October 2006, 07:08 AM   #126
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Hi

I first want to apologise because I made a huge mistake and my previous listening tests are worth nothing.
I accidentally swap the Ė output with Ground so all my listening where unbalanced out.
What I did yesterday where some listening with balanced out.
And try to get rid of the overshoot on some capacitive loads.
I put 100 Ohm at the output that didnít help.
Then I tried 2,2pf 4,7pf and 10pf on the 150k feedback resistors.
4,7pf and 10pf is enough to get rid of the overshoot but no capís sounded best.
I did comparison between the Theta Six-Shooter and the UGS with SACD.
Had a slight favour for the UGS.
Now I go back to a gain of 5 and do the listening test again.

Rob
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Old 10th October 2006, 08:31 AM   #127
peufeu is offline peufeu  France
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Hehe, that's the risk with balanced : too many wires, it feels like defusing a bomb... now shall I cut the green ? or the red ? argh.

Anyway I'm interested in your results !

While I like the idea of SuSy I see two disturbing things in this circuit :

- It behaves like an inverting amplifier ; ie. the input signal is current really ; to be able to input a voltage, a resistor is added at the input which converts the input voltage to current via the input virtual ground. This is well, but it makes the output impedance of whatever drives this a part of the problem. Note that the balancing of the differential pair is important (as per Self) so, if you drive this baby unbalanced, you should not connect the unused input to ground, but rather to an impedance equal to the impedance driving the other side.

- There is no buffer at the output ; open loop output impedance is quite high (basically equal to the Rout resistor) so it relies entirely on feedback to control this. What about adding a simple buffer at the outputs ? (an emitter follower biased by a CCS would do nicely).

When I have time I will definitely experiment with this clever circuit...
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Old 10th October 2006, 08:50 AM   #128
EUVL is offline EUVL  Europe
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Rob,

How about a frequency response plot from say 20Hz to 2 MHz ?
I find it useful to help to determine the feedback cap value.

Groeten,
Patrick
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Old 10th October 2006, 10:19 AM   #129
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Hi UEVL

The sweep looks nice from 20Hz to 1Mhz with a 750Khz roll off .
If you put a capacitive load you need compensation.
But when you do it sounds worse in my opinion.

Rob
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Old 10th October 2006, 02:15 PM   #130
EUVL is offline EUVL  Europe
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If you have no stability issues, then don't put caps on the feedback.

We do, however, have good experience with polystyrene and silver mica caps on the feedback (for power amps). Polystyrene actually preferred.


Patrick
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