Hi Andreas,
You have lived a sheltered life then.
There are far worse amps out there. However, I seldom see a properly repaired amplifier. Almost never a BGW750x. You may have never heard one running properly.
Latala,
The output stage of a BGW750 is no worse than any other amplifier of the time. They are all higher feedback designs. That was the state of the art back then. It's tricky to draw it out, but they actually went as far as to balance the currents in the predrive stage. What is the voltage on pin 6 of the LM318? It has to be less than 100 mV. The output DC offset will be good as long as the op amp is working.
-Chris
You have lived a sheltered life then.
There are far worse amps out there. However, I seldom see a properly repaired amplifier. Almost never a BGW750x. You may have never heard one running properly.Latala,
The output stage of a BGW750 is no worse than any other amplifier of the time. They are all higher feedback designs. That was the state of the art back then. It's tricky to draw it out, but they actually went as far as to balance the currents in the predrive stage. What is the voltage on pin 6 of the LM318? It has to be less than 100 mV. The output DC offset will be good as long as the op amp is working.
-Chris
I own three BGW750, I have copied them built about 10. This was 1980 something. Workhorses during touring with PA. Never broke down. My own had toroid transformers that made them carriable when carrying three at the time. Later I built a copy of Dynacord 1000W amp, with symmetrial, discrete right through, design. Very low open loop gain about 2000. These sounded absolutely great in every sense compared to the BGWs. Nowadays I have my own patent on power amps and they are a completely different story, welcome at:
www.tunerlux.com
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No no no, it is not constant, thenit wouldn't be a novelty.
The lower cirquit maintains at least ca 1V over the diodes at all time, thereby the upper transistor controls the output at all times, ie class A.
The resistor in parallel to the diodes is 10R ca, giving a Iquiscent of ca100mA, no unneccesary heat. Bingo, no heat and class-A.
I have built about 10 of these, and they sell for $2000. Most customers listen two songs, ask the price and buy, nice people.
Andreas
The lower cirquit maintains at least ca 1V over the diodes at all time, thereby the upper transistor controls the output at all times, ie class A.
The resistor in parallel to the diodes is 10R ca, giving a Iquiscent of ca100mA, no unneccesary heat. Bingo, no heat and class-A.
I have built about 10 of these, and they sell for $2000. Most customers listen two songs, ask the price and buy, nice people.
Andreas
Why will they sound bad you should be mpore precise with your answer !
Tha amplifier does not sound bad ! it is just not the best I have heard
But bad no way!!!
How will people learn unless you are more specific
For instance the comments an setting up the output stage have been very usefull very good food for thought.
And did you ever try to polarise the capacitors did they sound better?
Regards Trevor
Tha amplifier does not sound bad ! it is just not the best I have heard
But bad no way!!!
How will people learn unless you are more specific
For instance the comments an setting up the output stage have been very usefull very good food for thought.
And did you ever try to polarise the capacitors did they sound better?
Regards Trevor
Dielectric absorption.
http://home.comcast.net/~wjungieee/wsb/Picking_Capacitors_1.pdf
http://home.comcast.net/~wjungieee/wsb/Picking_Capacitors_1.pdf
Yes DC bias helps, but the supply must be dead quiet or it injects noise/hum into the circuit.
http://home.comcast.net/~wjungieee/wsb/Picking_Capacitors_1.pdf
http://home.comcast.net/~wjungieee/wsb/Picking_Capacitors_1.pdf
Yes DC bias helps, but the supply must be dead quiet or it injects noise/hum into the circuit.
IMHO amp with op-amp must be prone to TIM, transient iinter modulation distorsion. Matti Ottala was the inventor of that distorsion measurement. He devices that raw gain is distributed and that interstages have more headroom. If you look upon the BGW setup, first an OP then a grounded emitter stage with no defined load.
During an output transient the intermediate stage will be blocked due to overload a the output can't react fast enough to the input, there is always a limit on bandwith. The grounded emitter stage will stay saturated for a short but influential time.
Andreas
During an output transient the intermediate stage will be blocked due to overload a the output can't react fast enough to the input, there is always a limit on bandwith. The grounded emitter stage will stay saturated for a short but influential time.
Andreas
The LM318 can slew at 70V/µS, the Jung criterea for slewing induced distortion is a minimum of 0.5V/µS per peak volt of output.
That would mean only a 40V/µS slew rate would be adequate (for the 80V rails).
The second voltage amp is plenty fast too. Earlier ones used the 2N3440/2N5415 pair, later they used the MJE340/350 pair.
The outputs were the 4Mhz epitaxial 2N3773, later the same die family became the basis for the MJ15024/25 (see the ON Semi curves, they are shown on the same data sheet).
That would mean only a 40V/µS slew rate would be adequate (for the 80V rails).
The second voltage amp is plenty fast too. Earlier ones used the 2N3440/2N5415 pair, later they used the MJE340/350 pair.
The outputs were the 4Mhz epitaxial 2N3773, later the same die family became the basis for the MJ15024/25 (see the ON Semi curves, they are shown on the same data sheet).
Absolutely right and that is the problem. If the 318 slew 50V /us
its signal is amplified in next stage 100x that would need the power stage to slew 5000V/us. It doesn't, so the input signal of the 318 increases and that will finally overload the next stage.
I have seen it, it is easy so see. Insert 10kHz sq wave hook up the scope att the intermediate signal, and you will see big voltage swings that make the voltage amplifier transitors saturate.
Andreas
its signal is amplified in next stage 100x that would need the power stage to slew 5000V/us. It doesn't, so the input signal of the 318 increases and that will finally overload the next stage.
I have seen it, it is easy so see. Insert 10kHz sq wave hook up the scope att the intermediate signal, and you will see big voltage swings that make the voltage amplifier transitors saturate.
Andreas
Yes I am aware of TID however the uniy uses a fast op amp and provided that the input is slew rate limited surely its no problem thew siganl then goes to an active VAS stage where the VAS transistors work in unison to swing the output transistors quite a fast option I think.
I am no particular advocate of the BGW but its not fair top say its a bad amp !
I have always favoured amplifiers with local feed back loops and then a small overall feedback loop
Band width should then be limited on the input so as to minimise TID and slew rate distortion etc
However a large no of amplifiers go the way of the BGW and seem to be ok!
tha main problem to my mind with BGW type amplifiers is the the GM of the output stage changes significantly when the Class B boosters turn on
All though I should imagine its not a great audio problem at the volume levels that the amp is played at !!!
As for the input capacitor it seems to be a tantalium device ! We used to say was ok even though there is no polarising voltage.
Or are you referring to the feedback capacitor bottom of the feed back network , if thats the case then almost all amplifiers made can be accused of that error
Regards Trev
I am no particular advocate of the BGW but its not fair top say its a bad amp !
I have always favoured amplifiers with local feed back loops and then a small overall feedback loop
Band width should then be limited on the input so as to minimise TID and slew rate distortion etc
However a large no of amplifiers go the way of the BGW and seem to be ok!
tha main problem to my mind with BGW type amplifiers is the the GM of the output stage changes significantly when the Class B boosters turn on
All though I should imagine its not a great audio problem at the volume levels that the amp is played at !!!
As for the input capacitor it seems to be a tantalium device ! We used to say was ok even though there is no polarising voltage.
Or are you referring to the feedback capacitor bottom of the feed back network , if thats the case then almost all amplifiers made can be accused of that error
Regards Trev
Hi Guys,
The statement was that the BGW750 was the worst amp that Andreas had heard. That might be true, but I commented that there were far worse amps out there. For the time, the 750 was actually pretty good as a pro amp. I'd rather that than a Bryston 4B.
Are there problems with the BGW750? For today's HiFi amp - yes. For a good touring amp - minor (they are heavy). I think they are still a worthwhile amplifier. There are still new designs that sound far worse than a BGW750.
I haven't hung a 'scope in the Vas (careful with the probe capacitance), but I'll try with the next one I see. But the amp itself doesn't seem to have a problem with speed. Remember that high level 10 kHz signals do not happen in the wild unless there is a problem. I think there is an input filter on that one as well, so I would expect some rolling off of the edges of a square wave. In short, that is not a realistic test for the BGW750. Try a sine wave limited to realistic levels and have a peak. Keep in mind that for a sound re enforcement amp, it's pretty darn good.
-Chris
The statement was that the BGW750 was the worst amp that Andreas had heard. That might be true, but I commented that there were far worse amps out there. For the time, the 750 was actually pretty good as a pro amp. I'd rather that than a Bryston 4B.
Are there problems with the BGW750? For today's HiFi amp - yes. For a good touring amp - minor (they are heavy). I think they are still a worthwhile amplifier. There are still new designs that sound far worse than a BGW750.
I haven't hung a 'scope in the Vas (careful with the probe capacitance), but I'll try with the next one I see. But the amp itself doesn't seem to have a problem with speed. Remember that high level 10 kHz signals do not happen in the wild unless there is a problem. I think there is an input filter on that one as well, so I would expect some rolling off of the edges of a square wave. In short, that is not a realistic test for the BGW750. Try a sine wave limited to realistic levels and have a peak. Keep in mind that for a sound re enforcement amp, it's pretty darn good.
-Chris
I have at last finished the repair to my BGW
And I am in fact listening to it as I type up this report!
How does it sound ? Well very transistor !!But that is only to be expected for a unit designed at this period
The assistance you have all given me has been put to good use
The op amp rails on mine are at +/- 10 volts
The LM318 opamp was replaced with a plastic case 8 pin dil version
And i can not tell via listening between channels
between the metal /plastic can
AsI am using the amp with 8 ohm spk and its only a domestic situation I am thinking of disconnecting the cooling fan due to the noise it makes any thoughts re this?
Bottom end is quite good very tight well damped
mid is a bit edgy but the amp has only been on for 1 hour
speakers are a pair of B+W dm2
I am very much into 70s retro
The preamp is a Harmon Kardon HK930
To be honest i prefer the sound of the HK power amp its smoother
Once again Thanks lads
Next restoration a phase linear 400B
I must be a glutton for punishment
And I am in fact listening to it as I type up this report!
How does it sound ? Well very transistor !!But that is only to be expected for a unit designed at this period
The assistance you have all given me has been put to good use
The op amp rails on mine are at +/- 10 volts
The LM318 opamp was replaced with a plastic case 8 pin dil version
And i can not tell via listening between channels
between the metal /plastic can
AsI am using the amp with 8 ohm spk and its only a domestic situation I am thinking of disconnecting the cooling fan due to the noise it makes any thoughts re this?
Bottom end is quite good very tight well damped
mid is a bit edgy but the amp has only been on for 1 hour
speakers are a pair of B+W dm2
I am very much into 70s retro
The preamp is a Harmon Kardon HK930
To be honest i prefer the sound of the HK power amp its smoother
Once again Thanks lads
Next restoration a phase linear 400B
I must be a glutton for punishment
Good point!
I havent changed any electrolytics at all!!
Do you have any recomendations in this direction
I am sure that you are correct re these requiring replacement
Which would you chooseThe amp will not be gigged so i dont mind tuning it for domestic use with a 8 ohm load
the speakers that I am currently using are B+W DM2
regards Trev
I had this brush for forty years
6 new heads and 3 new handles
I havent changed any electrolytics at all!!
Do you have any recomendations in this direction
I am sure that you are correct re these requiring replacement
Which would you chooseThe amp will not be gigged so i dont mind tuning it for domestic use with a 8 ohm load
the speakers that I am currently using are B+W DM2
regards Trev
I had this brush for forty years
6 new heads and 3 new handles
I have purchased a copy from the states ! It is in transit as I write this. I can scan when i get it is this ok re copy right ?
I am still interested in its protection circuits and there are a few transistors etc that i can not see their function.Has anyone re biased the output stage into AB instead of AB-B
Also the fan is a pain and i may fit a autotemp control
I am aware its a dinosour bnut it what i want to play with at the moment
Will post schematic when i can all though due to house decoration my main PC workstation is out of service so there will be a delay!
Regards Trev
I am still interested in its protection circuits and there are a few transistors etc that i can not see their function.Has anyone re biased the output stage into AB instead of AB-B
Also the fan is a pain and i may fit a autotemp control
I am aware its a dinosour bnut it what i want to play with at the moment
Will post schematic when i can all though due to house decoration my main PC workstation is out of service so there will be a delay!
Regards Trev