BA283 questions

I'm building a Solid state Pultec (a la Steffen) and I have some questios about the BA283 part.
When I meassure at point A for DC at power up it jumps to 12V and then in 3 seconds or so to 6V and then after a minute it settles at 0,008V.When settled I have 13V at point B.
Then, when I power down it jumps to -4V and after 30 seconds it's back at 0V.
Is this normal behaviour for a BA283?




I'm asking because I want to use a Haufe AKM300 opamp buffer/transformer as output stages and it does not seem to like this 12V peak at power up.
When I connected the BA283 straight to a 5532 output buffer (like in steffen's design ) it works fine but when connected to the Haufe it seems like the Haufe starts oscillating or something like that, audio passes thru but it is distorted and it has this ramped tremolo effect (soft > loud > soft > loud etc.)

I don´t think it´s a normal behavior. Try changing the tantalum output cap...

> When I measure at point A for DC at power up it jumps to 12V and then in 3 seconds or so to 6V and then after a minute it settles at 0,008V.When settled I have 13V at point B. Then, when I power down it jumps to -4V and after 30 seconds it's back at 0V. Is this normal behaviour for a BA283?

It is perfectly normal for a cap-coupled single-ended output like this.

There are slow-start ways to reduce (not eliminate) this, but....

> ramped tremolo effect

You have an AWFUL lot of R-C feedback loops going. Four around the 283 stage. Six capacitors??? This topology is prone to motorboating anyway. The many caps, the 18K feedback, and the variable source impedance almost ensure it will find some way to putt-putt. What is this stage really supposed to do? You can get gain in the diff-input stage.

Thanks PRR. I think I will try the slow start way and if that does not work I'll just put aside the Haufe trannies and use a opamp as output buffer...
Any sugestions on how to make a slow start?


[quote author="PRR"]What is this stage really supposed to do? You can get gain in the diff-input stage.[/quote]

It's used for make up gain after a passive EQ stage.
I took it from Steffen's website.

As I stated earlier, when I connect the BA283 to a opamp output buffer it works fine.
It's just that I want to use those Haufe trannies.
I should stop buying junk on Ebay.

Any sugestions on how to make a slow start?

Click to expand...

The simplest way I can think of is to insert a 100 ohm between your +24 V rail and the 100 nF decoupling cap. Then parallel the 100 nF with a 6800 uF, this should slow things considerably. The values are just guessed, you'll have to try a bit.

Samuel

Tanks Samuel, I'll try that and if the start-up problem stays I'll get rid off those Haufe trannies and stick to Steffen's design (opamp in/output stage).