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Old 20th April 2004, 11:09 AM   #1
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Smile Please,can you improve this amp, let me know how

This amplifier is a Brazilian made Nashville unit.... i assemble and i am still using....but i fell something strange with the bass... i think it is precise... to much to my personal taste.... so if you can make it better, or different , in bass response....please, let me know.

This unit was made around a thousand.... now a days is still driving "trio eletrico".... electric trio is a truck band.... a mobile music machine.... Scania Vabis heavy trucks, 10 amps inside, air conditioned, 50 speakers surround truck... upside the musical instruments, microfones, switchers, mixers and all samplers and is making parties around this country.... two states visited in a month.... all the country knows them very well an Coke is the owner of the bigger one!.....heard by more than 150 milion citizens that lived in big towns.....others in jungle!

So it is hardly tested..... may have a mistake in schematics, this unit is made early seventies....1978 to be precise.

I have multisim....but i do not trust entirely on it.

Many thanks in advance the attention and time spent with me.

Carlos is the destroyer X
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Old 20th April 2004, 11:18 AM   #2
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Unhappy Complementary informations

I use 45 volts plus and minus..... normally this unit uses 62 plus and minus.

The input attenuator can be supressed..... is already cutted!.... input in 1K5 resistor.

Output impedance used is 8 ohms

Transistor with a floating 1K8 resistor is TIP41/TIP42...... the following transistor is also the same pair...... the output i used 2SC2922 and 2SA1216 by Sanken..... TWO PAIRS.

Factory used transistor metal can transistors alike 2N3055..... was some MJ models.... i do not remember 15004 and 15005...not sure.... a complementary heavy unit.

Those Sanken is around 250 volts maximum, 16 amperes maximum and 150 Watts maximum heat transfer..... gain is around 130 for NPN and 140 to PNP (my units).

Do you think a bias multiplier... a transistor to controll the bias will be better?..... Why?

Diodes is inside heat sink.... all them from bias.

If i forgot something, please, let me know.

Carlos
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Old 20th April 2004, 04:20 PM   #3
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i am just a beginner, so what i say might be stupid (so forgive me ) i dont get the point of this amp..if youre going to go to the trouble of having mirror Diff pairs..why not have active current sources and current mirrors?...

wouldnt it also be more desirable to have a cascode connection for the VAS
also what is the point of having a diode Vbe???...why not use the conventional transistor Vbe multiplier?...just a thought..
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Old 20th April 2004, 08:01 PM   #4
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... as you are suspecting errors in the schematic: I feel the 22pF at the lower transistor of the VAS should be placed from base to collector instead of base-emitter…

Also I am hoping that you use the C-types of the TIP 41 etc.
A and B types will struggle with voltage rating.
http://pubpages.unh.edu/~aperkins/pd...ices/TIP41.pdf


As you run the circuit with 45V instead of 62V all your biasing will be less
Than in the original design. You could come closer to the original if you decerase both 56k (from differentials to rails) down to 47k or 39k.
Please note this will also affect the bias of the output stage…

Possible improvement of PSSR, may result in improved hum, if necessary:
You could split the 56K (well or whatever you chose 47k…39k) into two
series resistors.
I.e. 10K to the rail + 33k towards the differential. From the center of both resistors
put 100uF to ground. So your bias of the differentials will get rid of the supply hum/noise.
Best solution of course would be to use some decent current sources instead of a simple resistors.

Next point is the bias of your output transistors:
I have doubts that the green LED + 3 diodes will compensate the temperature drift of the 2 x 3transistors = 6 diodes = 12mV/°C.
Also the bias level itself might be low or zero?
Zero would explain that you have no issues with temperature stability of the bias of the output transistors.

Which bias current in the output transistors do you measure.
…which values of emitter resistors do you use for your Sanken types and which DC voltage drop do you measure across each emitter resistor?

If this bias is to low, you will have a big influence on the sound.
Suitable values are 50 mA per pair of transistor.
If you have two pair this will generate about 2W constant losses in each transistor.
….makes 8 W losses in a Mono amp, 16W for stereo….

If you fear to design and adjust a bias multiplier, you could try this approach:
Pick 6 TIP41. Use each of it as a diode, simply by connecting the base to the collector. Then connect these “diodes” in series. This series connection of 6 diodes must be mount on the heat sink. This diode assembly would replace the series connection of the green LED and the three 1N4148.
Now try to adjust a proper bias by testing different values of the emitter resistors for the sankens. BUT BE CAREFUL. You should start with high values for the emitter resistors! May be around 2 Ohms for each Sanken. Then measure the bias current and adjust the resistors so that you get a reasonable bias current and keep observing the temperature of the heat sink…..

A bias multiplier would probably enable you to adjust the bias more comfortable and enable you for lower emitter resistors. But the lower values you will use for the emitters, the more accurate your adjustment and temperature compensation must be.
…lot of good opportunities to burn the expensive SANKENs during this….

Why is a proper bias necessary?
Without bias the output impedance of the amp will be very nonlinear for small output currents, resulting in high crossover distorsions.

For proper current bias of the output stage, you typically give a DC bias acrross the basis inputs. This DC voltage bias must give the base-emitter-voltages for the output stage ... + some millivolts more which you will see across the emitter resitors. Temperature compensation of this bias is very important, because some millivolts more DC bias voltage at the basis inputs of the output stage will give also some millivolts (little less than at the basis) across the emitter resistors. If you now have low values for the emitter resistors then, some millivolts will already make a remakable change in bias current......


Good Luck
Markus
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Old 21st April 2004, 01:42 AM   #5
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Thumbs up Thanks Markus... the Chocoholic

I will read tomorrow with time enough to reflections.

thank you..... i will return to you tomorrow... with questions maybe.

Carlos
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Old 21st April 2004, 08:52 AM   #6
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Smile Markus, chocoholic

Sending you new schematics and three attachments with text.
Carlos
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Old 21st April 2004, 08:55 AM   #7
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Thumbs up Markus answer... to chocoholic

This is the second of three text attached as image... the number for will be new schematic made considering you told me.

Carlos
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Old 21st April 2004, 09:00 AM   #8
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Thumbs up Mr. Markus, the Chocoholic

The text as a image, attached, number three.

Carlos
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Old 21st April 2004, 09:06 AM   #9
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Smile Mr Markus, The chocoholic

Markus..... here is the new schematic..... include VBE multiplier.... i forgot to put the condenser between 39K resistor that is in the first stage base..... i will divide this resistor in two unit the way you said.... 12K plus 27K...... and a condenser from those resistor junction to earth.

You will see a 470 Microfarads Condenser without effect...near the audio signal generator (virtual)..... left side and up the schematic..... i will not use it.

Please...let me know if you find something else.

In a week i will fix everything and tell you what i "feel" related to sound changes.

Carlos
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Old 21st April 2004, 11:08 PM   #10
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Hi Carlos!

I think you got my points.
Just two further thing which I noticed:
- The transistors of the VAS with the 22pF are shown as BC546
and BC556. BC 556 and BC546 are 65V types, but in the application they may see up 90V (+/-45V... full swing). So I am quite stunned that your amp works since years with such underrated transistors. I would expect 100V types or higher in the VAS. Especially if I think that the original amp was running with more than +/-60V, I would expect 150V types..... ???!
- Your bias multiplier shows 390 Ohms + 2700 Ohms from base to collector and 47 Ohms between base and emitter.
The idle current of the VAS may be between 4mA and 5mA, this
will not match to the resitor values of the bias multiplier.
Your transistor of the bias multiplier will remain turned off and allow
a horrible high (and probably destructive) bias for the output stage.
If you have about 3000 Ohms between base and collector then
suitable values for the resistor between base and emitter would be
around 500 ohms... 600ohms. I would propose to use 560 ohms and split the resistor from base to collector in 1.8k fixed resistor plus 1.8 k potentiometer. For first turn on you should put the potentiometer to 0 Ohms. This will make the transistor of the bias multiplier conductive and result in nearly no bias current of the output stage..... Then you can slowly increase the value to the desired value.... hope this works, it is just some rough estimation by hand!

Good Luck and thanks for the infos about the insight of the SANKEN!
Markus
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