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Old 29th September 2006, 12:46 PM   #81
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Hi Cheff

Yes I used a different transformer else I must have buy one.
I don't think the 3 volt difference is a problem

Rob
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Old 29th September 2006, 12:51 PM   #82
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Hi Rob,

No, 3 volts won't hurt. You can even more decrease the supply voltage,
but you'll have to take care of the cascode biasing voltages to get
9-10V at the bases.

BTW, have you measured the DC offset ?

Cheers,
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Old 29th September 2006, 01:20 PM   #83
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Default Re: Output Impedance

Hi Willus,

Quote:
Originally posted by z_willus_d
Cheff, as for lowering the value Ratio on the resistors in my attempt at adding Tinoís BJT bias circuit, that doesn't work for me. With the current values I simulate ~4.5mA through the JFET and the Vgs is ~0V. I did note in the 2sk170 datasheet that a Vgs of -400mV gives a better/flatter Ids curve, but I wasnít able to obtain the correct 10V/-10V split with that Vgs. Lowering the resistor value ratio for higher currents threw off the balance of the voltage divider.
Err... I was not speaking of Tino's circuit in this case... Sorry for
misunderstanding... In Tino's case, you'll have to get the b(i)ase
voltage of the folded cascodes close to the Jfets drain voltages
(just a Vbe apart). So adjust the CS and resistors to get it...

Quote:
Nevertheless, isn't 4.5mA through the JFET nearly ideal? That should be more than enough to overwhelm the minor Base currents in the uA range, right? I agree that this CS adds little benefit other than PSRR, but it did seem to help balance the base-voltages, whereas with the simple resistor divider I get a slight imbalance of base bias voltage between the NPN and PNP transistor nodes.
Base voltages for the cascode on "my" version are not as much critical
as in Tino's. A slight imbalance can be lived with - IMHO-, it doesn't
have adverse effects on sound nor on offset.

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Old 29th September 2006, 01:28 PM   #84
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Hi Cheff

Quote:
Originally posted by CheffDeGaar
Hi Rob,

No, 3 volts won't hurt. You can even more decrease the supply voltage,
but you'll have to take care of the cascode biasing voltages to get
9-10V at the bases.

BTW, have you measured the DC offset ?

Cheers,
I can keep it below 1mv and its very stable it changed a little with different input voltages that's all.

Rob
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Old 29th September 2006, 01:31 PM   #85
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Thanks
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Old 3rd October 2006, 02:46 PM   #86
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Hi

Yesterday evening we did a shootout between the UGS Module from Cheff and a Balanced Tube preamp from myself.
I also did a shootout between the Tube Preamp and a Theta Casablanca III and a Six Shooter which I own.
I can tell that the tube amp wins in the midrange but misses a bit of air on the top end against the Theta.
I did some changes on the tube preamp and find some things in that area and have tot do the shootout again but I think itís now in overall favour for the tube.
Back to the UGS Module in short it canít beat the tube amp but it sounds very well no harshness or a typical transistor sound.
In detail nothing is wrong with the Module but itís not so musical as the tube amp.
Itís very difficult to explain but with the tube amp you get goose bumps or how do you call this and with the UGS Module not .
The Soundstage is smaller and has less air than the Tube.
With the module is nothing wrong and you miss nothing till you change.
I think also that the tube amp is a little more dynamic and maybe we can change that by lowering the overall feedback so to have more gain.
Also I leave it on al day before listening and the offset was always below the 1mV.
So I think we can also increase the value from the 1k resistor to ground and listen what those 2 things bring.
I didnít use output caps or input caps and I use a 50K alps potentiometer at the input not output.
The input was unbalanced and output balanced.
I listen to a Denon A1XV an aleph kind of amplifier and my own build speakers.
Maybe not what you want to hear but thatís it.

Rob
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Old 3rd October 2006, 02:49 PM   #87
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Here is another shot from the 2 preamps side by side one is a bit empty but who cares.

Rob
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Old 3rd October 2006, 04:06 PM   #88
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Quote:
Originally posted by Rob Dingen
Here is another shot from the 2 preamps side by side one is a bit empty but who cares.

Rob

in case that you have unbalanced input to ugs ,then entire virtues of susy weren't present

don't take me wrong,tube amps and preamps are my love too,but UGS is (at least how I see it) constructed for full balanced

for other combinations Papa have other more romantic things,if we chase romantic -goose bumps-as you say
I always do
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Old 3rd October 2006, 04:15 PM   #89
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thanks for sharing!

looks like you used UGS in unbalanced mode?
any chance of trying balanced?

also curious if any way to try a version of UGS that has no overall global feedback? i'm curious about opinons on that ...

mlloyd1
edit: ooops, i see zen mod already commented on balanced and balanced operation for UGS ...
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Old 3rd October 2006, 04:32 PM   #90
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Quote:
Hi
Yesterday evening we did a shootout between the UGS Module from Cheff and a Balanced Tube preamp from myself.
I also did a shootout between the Tube Preamp and a Theta Casablanca III and a Six Shooter which I own.
I can tell that the tube amp wins in the midrange but misses a bit of air on the top end against the Theta.
I did some changes on the tube preamp and find some things in that area and have tot do the shootout again but I think itís now in overall favour for the tube.
Back to the UGS Module in short it canít beat the tube amp but it sounds very well no harshness or a typical transistor sound.
In detail nothing is wrong with the Module but itís not so musical as the tube amp.
Itís very difficult to explain but with the tube amp you get goose bumps or how do you call this and with the UGS Module not .
The Soundstage is smaller and has less air than the Tube.
With the module is nothing wrong and you miss nothing till you change.
I think also that the tube amp is a little more dynamic and maybe we can change that by lowering the overall feedback so to have more gain.
Hi Rob, thanks for reporting.

What's the gain of your tube pre, and the input sensivity of your amp ?
I confess I'm a bit puzzled about dynamics, since I didn't feel a lack of,
rather the opposite , but I'm not as experienced (and equipped)
as you are.

It could be related to the bandwidth you measured. I'm not quite sure
the perfboard layout is optimal in terms of parasitic capacitance,
and thus BW limiting, and this could impact the dynamics by slowing
transients. I will try to grab a decent generator and try to measure
BW on my modules with pcb, or at least see the shape of squares
signals at 100kHz...

And the same goes for soundstaging and air... I felt the opposite,
but didn't compare with a tube pre, just my old BoSoZ and the
"no preamp" solution ...

But when it comes to goose bumps, that's only a personal question
of culture, education and taste And that was exactly the opposite
when a friend of mine switched from a tube pre to the ugs
He was much more thrilled by the ugs than by the tube...

Quote:
Maybe not what you want to hear but thatís it.
No need to feel sorry I never meant it was the ultimate preamp,
and it surely can be improved. But the overall impression I retain
from what you wrote is that you miss the perceived "magic" of the
tube pre,otherwise nothing's really wrong...
And it's very hard to compete with magic

Anyway, warm thanks for taking the time to build and test it.
Just tell us how it goes with a higher Rout and a gain modification.

But for the moment, we're at deuce : amongst the two that have
built and listened to the ugs, one love it, the other not...
Can't conclude

Cheers
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