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Old 12th April 2015, 04:47 PM   #1
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Algar_emi's Avatar
Join Date: Oct 2002
Location: Canada, Qc
Default My UGS UP Amplifier

Hi. I was licky enough to grab a set of the UGS UP Amplifier from the Swap meet a few weeks back. See the original project thread at:
UGS Power : it's U.P.

I've being working on the project since then. I but together my own Parts Lists with the current parts and price available from Mouser and Digikey.

All pcb are almost completed. I'm waiting for the IRF240, IRF9240 matched sixtet to arrive. In the mean time I was searching for a reliable source of 2SA1209/2SC2911 used for the front end UGS cascode. These transistors are long time discontinued and I was quite worried with the full load of counterfeit from China. I'm quite happy (and relieved) to have found a reliable source in the US at a very reasonable price. There is still honest shop around, good for us Diyer

When I got the 2SA1209/2SC2911 I measured their HFE, all looking good. Then I used my curve tracer and compare the response curves with the datasheet. Just perfect almost bang on with the specs. I then matched them by their HFE.
The top and bottom cascode are matched (Q5-Q8), same for Q9-Q12. I also matched both BC550 and BC560. Don't know if this is really necessary but it certainly couldn't hurt with such a balanced design.

First I tested the auxiliary circuit on the main amp PCB, both front end +/-30V regs and the balanced DC output protection circuit. All were working perfectly.

Then I installed the 2SA1209/2SC2911 on sockets for testing on the main amp pcb. I installed also sockets for the 2SK389/2SJ109 so I can matched them even closer if I experience some circuit imbalance and I couldn't adjust the output DC offset.

To be able to test the front end without the final power MOSFET, I didn't installed the VBE multiplier Q13/Q14, and used the local feedback jumper. Rout was set at 10K for testing.

With some expectation I turn on the amp channel supplies. Just great I was almost right on for the output differential and absolute DC offset, nothing was burning and the front end was working just fine I guest the close matching of all sides help in this regard.

Check the picture of my test PCB. I also posted my circuit actual voltage readings. As expected all voltage readings are the same for both side of the circuit (X balanced design), hence I've shown the readings on just one side.

The specs I have so far with the Front-End only are:

Vin (BAL) = 100mv (1Khz),
Vout (SE) = 0.7V (2Vp-p), Gain: 17dB
Vout (BAL)= 1.4V (4Vp-p), Gain 23dB, (+6dB as expected, X100 specs 26dB)
BW (ref 1khz, -3dB) = 211Khz (with local feedback)
THD (1Khz) SE < 0.05%
_ _ _ _ _ _BAL < 0.005% (THD cancellation by X design, interesting...)
Noise (inputs shorted) < 0.5mv
Vin Max (BAL) = 1.9V (original X100 specs is 1.4V)
for Vout (BAL) = 26.7V

All things considered these are excellent specs
Attached Images
File Type: jpg UP_Front End_Test.jpg (170.1 KB, 205 views)
File Type: jpg UP_UGS_with Notes Test Results.jpg (171.1 KB, 203 views)

Last edited by Algar_emi; 12th April 2015 at 04:53 PM.
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