can anyone help me with this amp? - Page 2 - diyAudio
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Old 17th November 2001, 04:18 PM   #11
diyAudio Member
 
Join Date: May 2001
Location: Denmark
Default Have you modified your circuit ?

i don't understand germany, but im interested in building the amp. but There is some think i don't understand how come it can play 280w from only 1 par of transistors?

or have i misunderstand something (Germany :-)

I like the simple pcb and compact sice.
how does it sound . im going to use it for active monitors on stage.

jc dk.
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Old 17th November 2001, 05:33 PM   #12
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Join Date: Jul 2001
Location: Orleans, France
Hugo,

It's no simple way to predict what will occurs if you use an operational amplifier, preferabily of audio type, at the input of this amplifier, depending of layout, supply decoupling, etc. Anyhow, this will add some distorsions and phase shift, and the stability of the amplifier will be poor, because this will not cure the **reals** problems, which are excessive open loop gain, and insufficient decoupling, causing severe HF oscillation.

Again, it's more simpler to correct some design errors in the design : R1 must be equal to R5, one resistor of 47 ohms or so must be insered in the emitters of each BC556B, decoupling condensers of 1 uF or more must be connected between + 48 and output GND, and - 48 and output GND.

As usual, output wires must be as far as possible from input wires. Input wires must be shielded. No connexion must exist between input and output GND except through the PCB, the chassis must be connected to GND at one point, no more, etc...

This done, you must verify that it's no residual HF oscillation at the output, with an oscilloscope, whithout, an with signal. Idle current of output transistors (without signal) must be 50 mA or so, (=50 mV between emitters of this transistors).

Regards, P.Lacombe.
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Old 17th November 2001, 06:00 PM   #13
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Join Date: Jul 2001
Location: Orleans, France
Partyjups,

Calculations shows that this amplifier can deliver 250 W RMS into a 4 ohms load, and 130 W RMS into a 8 ohms load, with the specified +/- 48 V supply (stabilized).

But there is very low safety margin, as we can have seen in the fact that HF oscillation can destroy output transistors...

In my opinion, it seems more prudent to reduce supply voltage, by instance +/- 40 V, for 90 W output into 8 ohms. For 4 ohms loads, and 180 W output, it is necessary to double the output stages (and heatsinks).

Regards, P.Lacombe.
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Old 17th November 2001, 07:19 PM   #14
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Join Date: Jun 2001
Location: Flanders, Belgium
Exclamation I've contacted the designer

This is what the designer replied to my mail I've send to him:

First im want to say sorry, for my
bad english, and for the problems,
this amplifier makes.

I know this Problems.

It seems it can happend that the
amplifier swings with a high frequenz,
and unfortunately I could not make a
change in the Plan to prevent this
absolutely.

I only can say, that with a lower
voltage of +/-30 V this Problems
can not happen.

Please tell me, where you found the
280Watt.zip, because I want to write
mails, to everywhere, where this plan
is in the internet, so that they take this
plan out of the internet.

Perhaps you can trie to change C5
to 330pF. But I don't know, if this
solve the Problems.

Bye, Robert Sontheimer




Best regards,


HB.
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Old 18th November 2001, 08:12 AM   #15
diyAudio Member
 
Join Date: Nov 2001
Hello,

I'm the designer of this amplifier. My english is not so
good, but I want trie to expain some things, and awnser
some questions of this thread.

I buid this amp some years ago, and never had any problems
with it. But some other people in germany who build it, had
problems with sudden oscillation of the amp.

This problem seems to have nothing to do with the input.
Ist's not only in case of no input.

Some months ago I tried to solve the problems with some
changes in the plan 280Watt.zip, and I have seen, that the
zip-file you use, is the oldone. The new zip-file with the
changes you can download there:

http://people.freenet.de/r.sont/280Watt.zip

But the changes in this new 280Watt.zip unfortunately
are no safe solution, to prevent this oscillation
absolutely.

Now I want to say somethin to this thread:

> I suggest to connect in series with each emitter of the
> input differential pair a small resistor (33...68 ohms)
> in order to keep the open loop gain at usual value.
> This will improve the stability of the amplifier at high
> frequencies.

Yes, I think perhaps this can be a solution.

> Also, R1 must be equal to R5, i.e. 8.2 kohms, for correct
> ac/dc balance of the differential amplifier.

I think this ist wrong, because if the current in T1 is
bigger, or the current in T2 is bigger, depends on the
amplification of T3, and this can be bigger or smaller.

> i don't understand germany, but im interested in building
> the amp. but There is some think i don't understand how
> come it can play 280w from only 1 par of transistors?

> or have i misunderstand something (Germany :-)

The used transistors TIP142 and TIP147 are very big,
but it's importent to put a fan directly on the kuehlkoerper
(sorry, I don't kwon the right word in english, but look at
the picture montage.pcx)

A second pair of transistors is not necessary, and can
not solve the oscillation-problems.

If someone now makes tests with changes at this amp
(for example emitter-resistors at T1 and T2) and
find a solution to prevent this oscillation-problems
reliable, I would like to know it. You can send me
an e-mail to r.sont@freenet.de

Best regards from germany,

Robert Sontheimer
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Old 26th November 2001, 08:07 PM   #16
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Join Date: Jul 2001
Location: Orleans, France
Robert,

You can see here :

http://users.ece.gatech.edu/~mleach/lowtim/instage.html

explanations about utility of emitter resistors in the input differential pairs. I use such resistors in all my desings since 10 years, and I have no more problems with HF oscillations. Furthermore, this will dramatically decrease transitory intermodulation distorsion.

Concerning doubling of output transistors, if you use 4 ohms driver as load, effective loading can be as low as 3 ohms at 400 Hz or so, causing intensity reaching 15 A peak. TIPs are specified 10 A continuous, 20 A peak, so safety margin is too low in my opinion. Because of this, I have suggested to double the output transistors in the case of 4 ohms load. This will most certainly have no effect on HF stability, I agree with this.

Regards, P.Lacombe.
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