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Old 28th September 2006, 06:01 PM   #71
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Hi

Forgot to tell.
The output impedance is 230 Ohm.

Rob
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Old 28th September 2006, 06:23 PM   #72
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Quote:
Originally posted by Onvinyl
what do you say would be the highest possible good sounding gain with this topo?
I have gotten as high as 80 dB open loop without special
effort (or load), but the quality is much dependent on the
application.
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Old 28th September 2006, 09:53 PM   #73
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Hi Rob,

Quote:
Originally posted by Rob Dingen
Hi

I build exactly like your schematics and did some measurements.
Only my power supply = +/- 21V and I use BC550 and BC560.
Gain SE in and Out 1.65
Gain SE in and Bal out 3.3
Bandwidth 760Khz -3dB
There is no overshoot.
Max amplitude SE = 20Vpp and Bal 40Vpp.
Distortion measurement is difficult because its balanced out and I don't have tools for that.
But when I measure one side its below 0,01% and second harmonics so Balanced should be lower because it cancel 2e harmonics.
I attach a picture from my scope with a square wave at 100Khz
Wow !
How quick, and what a nice job !

Concerning your supply, is it the schematic I posted ?

What about the DC offset ? Did you measure the bias of each stage ?

I'm a bit surprised by the max output amplitude, pretty close to the rail
But I only simulated it, and have no measurements, my signal generator
output not being high enough for the gain I chose...

But these are globally nice figures, IMHO at least
Do you happen to have a "real" distortion meter ?

Do you have a picture of the direct signal from your generator corresponding
to the USG output signal (100kHz square wave) ? Just to see if the
transitions are really rounded by the UGS... My generator begins to
exhibit some weakness after 20kHz (DIY...), so I really can't tell about
the HF behavior... But given the BW you measured, I think I should
blame it on the UGS (1/7th of the BW @ 100kHz), and not on the generator...

Quote:
And the picture of my board.
I only build one half.
Now I finish it and do some listening.
Again, nice and neat job.
Quite eager to read your listening impressions

Quote:
Forgot to tell.
The output impedance is 230 Ohm.
Balanced or unbalanced ?
How did you measure it ? I just have some doubts on the method for
assesing it... First sims I did gave around 500 Ohms, but I not so sure now.
Each method I chose gives different results... I'll have to try to get it
through the circuit equations... When I have time

Anyway, thanks for trying it, and for the measurements you made.
When do you think you can report back your listening impressions ?

Regards
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Old 28th September 2006, 11:32 PM   #74
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Default Output Impedance

Rob, et al., I too would be interested in the method used to determine output impedance. Cheff, how did you simulate it? Output into a known impedance with pass on frequency looking for attenuation? SE is twice the imp of BAL right?

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. 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.

Iím still waiting on the transistors to build a test board. Fairchildís sample system/website is the worst, slowest, and most useless (or user-friendly-less) system Iíve encountered in the industry.
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Old 29th September 2006, 01:38 AM   #75
spencer is offline spencer  Hong Kong
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Patirck,

Yes, I cannot know whether they are real or fake but at this price, there is no point for people to clone it. Actually in Singapore I only pay $0.08 for A970BL (higher hfe part) which is even cheaper than Shenzhen but the Singapore distributor did not carry any C2240. I check all the hfe and they are within spec. Is there a way to check whether they are fake?

So far I used some 2SK170BL from Shenzhen and I do not see any difference from those I get if from Singapore distributor. I did experience some fake IRF610 or IRF9610 before and at the end I have to buy from Singapore distributor which is at a bit higher price.

Jacco, thank you for your offer but I do not need more J109 at this moment. I hv 10 pcs more and is enough for me to build one more pre-amp.

Cheers
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Old 29th September 2006, 05:22 AM   #76
EUVL is offline EUVL  Europe
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> Is there a way to check whether they are fake?

You can run some current through with a function generator (triangular wave), measure base and collector current using an oscilloscope, and plot channel 1 vs ch 2. The slope gives you hfe, and you can see how constant hfe is with Ic.

The curve for fakes (that I measured) are not linear.


Patrick
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Old 29th September 2006, 09:34 AM   #77
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Hi Cheff

I build the same powersupply.
Bias jfet is about 3.8ma and output 7.6ma
And that bias with 1k to ground and you got the output swing
I have a distorsion analizer from Hameg not very hightech but good enough and distorsion is also at output to look at it on a scope.

Rob
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Old 29th September 2006, 09:42 AM   #78
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I attach a picture from the scope with 100Khz signal output and input.
The output impededance is measured with a 1K resistor between + and - output.
Input is - input to ground.
Then I take a thru RMS meter put 4 volt ac to the output of the preamp and then short it with the 1k resistor.
With the difference between the to value you can calculate the output impedance.
I hope to setup a listening test on Monday evening.

Rob
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Old 29th September 2006, 11:45 AM   #79
philbyx is offline philbyx  France
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Default Re: Output Impedance

Quote:
SE is twice the imp of BAL right?
Unbalanced output impedance is half the balanced one.

On simulation with Cheff's UGS, I have 125 Ohms unbalanced (between out+ or out- and Ground) and 250 Ohms balanced (between out+ and out-).

Philippe
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Old 29th September 2006, 11:51 AM   #80
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Hi Rob,

Quote:
I build the same powersupply.
OK. You use a different transformer, or is it a choice of yours to limit
the suplly voltage to 21 V ?

Quote:
Bias jfet is about 3.8ma and output 7.6ma
As expected

Quote:
I have a distorsion analizer from Hameg not very hightech but good enough and distorsion is also at output to look at it on a scope.
I wish I have this one If my math is ok, the measured distortion of
less than 0.01% gives less than -80dB, and that's the simulated range.

Quote:
I attach a picture from the scope with 100Khz signal output and input.
So I was right to blame it on the UGS Thanks for the picture.

Quote:
The output impededance is measured with a 1K resistor between + and - output.
Input is - input to ground.
Then I take a thru RMS meter put 4 volt ac to the output of the preamp and then short it with the 1k resistor.
With the difference between the to value you can calculate the output impedance.
I should have remember Thevenin's theorem Shame on me
A variation on the (open circuit voltage/short circuit current) classical
formula
I just tried it with the simulator, and it gives 233Ohms balanced,
and 116.5 Ohms unbalanced. Quite impressive to achieve such a
great agreement with measurements ! Even the gain figures are
the same you have (3.3 balanced). Too good to be true

Quote:
I hope to setup a listening test on Monday evening.
Rob
I just can't wait

Thanks for all the clarifications, and happy building.

Regards
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