Explendid amplifier designed by Michael Bittner, our MikeB

[dexter]

Mike,

I measured the following
r5/6: 1.001 and 1.001
R10: 0.347V.
R15/17: 0.367V and 0.385V

I'll calculate the mA into mV and strat probing

Can you suggest what other things I should measure?

The Dc offset is 0.1mV for both channel

Thak you for your help
best rgds,

[MikeB]

Hi dexter, your measured values are perfectly fine. This means input stage and vas are functional. So, the problem is in the outputstage / vbe-multiplier.
Try measuring the voltage T1: C -> E, base U5 to base U6.

Mike

[dexter]

HI mIke,

Many thanks for the reply.

I measured 1.5V between U5 and U6.

thanks mike and best rgds,

[MikeB]

Can you adjust these 1.5v up to 2.4v with the bias-pot? Have you accidently reversed the bd139 (T1)? It has e-c-b pinout.

[crt]

@ MikeB

in my symasym

the DC offset is still around 7mV for both channel, even I have change the differential with match hFE (I use cheap multimeter to match it). Is that OK?

And I use toshiba for output transistor. Have you try to put c5200 and a1943 with same parts (refer to version 5.3)?
only feedback caps is change from 22pf to 10pf

how to know if my symasym is oscilated? I don't have stuff like osciloscope

sorry for my BAD english, I hope you understand what i mean.

Thank you

[dexter]

Mike,

thanks for the reply.

I turned the pot and the voltage stays at 1.5V.

for bd139 vbe, I think the pins're correctly placed

I've checked all the tr w/ my dmm and they all seems ok.

best rgds,

[AndrewT]

Originally posted by crt
in my symasym

the DC offset is still around 7mV for both channel, even I have change the differential with match hFE (I use cheap multimeter to match it). Is that OK?

no.
There are three pairs of transistors that must be matched.
Using a DMM hFE tester with unknown base or collector current is not good enough.

You must match each of the pairs at the operational Ic as in the working circuit.
I also match the Vbe at the operational current. In fact I consider Vbe matching to be far more important than hFE in the matching.

[anatech]

Hi Andrew,



If you match transistors for beta from the same batch, the Vbe will also be matched. The reverse is not true. If you are trying to match parts from different batches .... why? It really is that simple.

Now, if you can match beta along several different emitter currents (close to collector currents if you want to get picky), then the match of Vbe will only affect DC offset by a small amount. The beta matching is crucial for AC performance as this is your error corrector. The most critical of all things to match. Try it with a good THD meter.

We really do agree on most things.

Hi crt,
Andrew is dead right on disagreeing with your matching method. Device temperature will greatly affect your readings. I designed a jig, and I'm sure most of us have "designed" that same jig. Basically, you build a differential pair with sockets for your two parts. Let the temperature settle and have the two parts touching each other. The ones that pass equal current are matched. I measure across collector resistors. You need matched resistors for the two base resistors (to common ground) and your two collector load resistors. Mine has variable current, but you can fix yours to 2 mA (1 mA per transistor) to match reasonably closely most situations. I also generally run collector voltages of 10 VDC. So this is easy and inexpensive to build.

-Chris

[crt]

Thank's anatech

just want to post my symasym

[IMG][/IMG]


still playing until now.

[AndrewT]

Originally posted by anatech
If you match transistors for beta from the same batch, the Vbe will also be matched. The reverse is not true.

I have found that matching hFE does not automatically give good or any match for Vbe. But maybe my group of one or two hundred are not from the same batch.
As a result of this finding, I match Vbe as close as I can manage using your jig which ensures matched Vbe and check for equal Ic in the balanced pair. I then read off the Ib from the base resistor voltage drop and try to get those within 5%.
I then go back to the identified possible pairs and remeasure with a zero value base resistor (temp short across the connections) to recheck the Ic values are within 1%. Many possible pairs fail this last test.