Old amplifier circiut

[ilimzn]

Originally posted by camt1
With no quiscent current bias and operating in class B i assume that the crossover distortion will be high.

You assume correctly.

Would i be right in thinking of the following values for the VBE multiplier...

A bit difficult to say anything about this since you failed to provide a schematic for the Vbe multiplier which would tell us which resistors R1 and R2 are.
In your schematic, the Vbe multiplier needs about 3x Vbe across it, and also an ability to start with lower than that, i.e. zero bias current (provides a safe setting for powering up the amp first time).
Try this: connect 2.2k across BD139 C and B. Then connect a 2k pot's outer contacts from B and E. Connect a 1.8k between the BD139 E and wiper of the pot. If you need a higher maximum idle current, you can reduce the 1.8k to a 1.5k.

A further possible improvement would be the inclusion of a Baxandal diode in the emitter circuit of Q7 (search this forum for details). This will require the Vbe multiplier to give approx. 4x Vbe maximum across it, so the resistor between wiper and E of BD139 should be reduced to 820 ohms to 1k.

Originally posted by phase_accurate
It looks as if this amp behaves like an Edwin amp where only the drivers conduct at low output currents. This correlates well with your observation of drivers being hotter than the output devices themselves.

Actually, no - the Edwin had a relatively high bias class A driver stage that was also conected via a resistor to the output. Similar to forum member Wavebourn's Swinik.

[darkfenriz]

Resistor at Q2 collector is a bit too high.

[Tube_Dude]

After the implementation of the Vbe multiplier, as Ilimzn told you , is a good idea to diminish the value of the resistors at the output from 1 Ohm to something like 0,2 , 0,33 Ohms.

Also, put a bypass cap across the Vbe multiplier circuit ...

[camt1]

hi thanks for the replies . I have modifed and installed the VBE circiut , i have set the voltage drop across the output transistor resistors at 10mv each . I removed one leg of one resistor and found that they are not 1R but labelled as W21 R10 this if memory serves me right means they are 0.1 reistance , which means that quiscent current is 100ma if im right, is this too high ? . Output transistors warm but not hot , have tested underload with no problems of over heating and appears to be stable. I have one further question , one channel is a lot quiter than the other with DC on the output of 350mvcompared to the other channel which is less than 10mv . I have checked over the boards and cannot find any differnces with voltages or components from one side to the other , does anybody have an idea what i should be looking for to make the gain the same as the other board. What values should be the feedback capacitor (at prensent its 22uf ) and the cap accross Q5 base collector (no markings to be seen ) thanks for any help
camt

[richie00boy]

The feedback cap is a bit too small at the moment, but increasing it will make DC offset worse. I do believe there is a fault with the channel showing more offset. >100mV is too much. Has the amp been repaired at all or subjected to any 'bad' conditions such as shorted speaker wires or pops or thumps e.g. by plugging/unplugging live sources?

[Tube_Dude]

Originally posted by richie00boy
The feedback cap is a bit too small at the moment, but increasing it will make DC offset worse.

The value of the feedback cap has noting to do with the offset voltage.

But the cap can be leaking and if so ,it can be the culprit of the offset issue...

Inspect also the input capacitor for leaking.......

[camt1]

Yes amp has been repaired at some time all trans except Q1.2.3 have been replaced , the only change being TIP32C used in place of orignal TIP30C (different hfe seems to be the only difference). Replaced i/p cap and feedback cap no change to DC offset or to lack off gain. Would BC448 be a suitable replacement for Q1.2.3 BC477 as there are the only orignal trans left and mayybe suspect.
thanks camt

[camt1]

thanks for the help replaced feedback cap with a 47uf from 22uf this seems to have fixed the gain issue
camt

[Tube_Dude]

Originally posted by camt1
Would BC448 be a suitable replacement for Q1.2.3 BC477 as there are the only orignal trans left and mayybe suspect.
thanks camt

The voltage ratings (80volts) are a little in the low side for your's +- 50 volts rails.

It can work, but with a 100 volts transistor you will have more piece of mind...

First try to swap the transistors from one channel to the other and check if the problem follow the change...

Here is the ratings of the BC448

http://www.datasheets.org.uk/specsheet.php?part=BC448

replaced feedback cap with a 47uf from 22uf this seems to have fixed the gain issue

The value of the capacitor ,as I have already told you ,don't have nothing with the gain in the audio band( only in the very low frequencies ~10Hz).
If the gain have changed , proved that the original capacitor was bad...

[camt1]

sorry should have explained amp is only being used to drive sub woofers from 90Hz down over , it is being fed from linkwitz rielly crossover and linkwitz transform circiut so it was only at low frequnecies that the lack fo gain was noticable .
camt