I use a signal generator and a scope to set up bias.
Turn up bias slowly until crossover distortion goes.
This can be as low as a few mA depending on the amplifier.
That is a good approach.
With mosfets I go to the other extreme. I tend to bias as high as my heatsinks will tolerate (around 50 degrees). However if there is no audible difference then I generally drop it to around 100mA per device.
Mosfets tend to like more bias than bipolars.
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50mA to 100mA should be acceptable.
For me I let the ear be the final judge in these things.
Read the datasheets, zero tempco for the K134/K135 is above the 100mA level.
The P-channel complementary J49 (J50) has zero tempco at a lower level, ~75mA.
Biasing the P- and N-channel differently, plus have 0Vdc at the output, will be rather tricky.
Old school biasing rule for AB Hitachi Lateral MOSFET output stages is 100mA per.
By the meter/scope, you'll get closer to the 125mA mark.
To night I have gone through this whole thread, if possible, to get a little wiser on the bias setting.
But I get a little bit confused. In one post it is referred to R20 (270 ohm) beeing the biasresistor, in other posts it is R23.
Is it to say that both resistors affect the bias?
Many of you have answered my question on "optimal" voltage drop across soursresistors.
The answers vary from 50 mA to 320mA.
Jacco wrote: Read the datasheets, zero tempco for the K134/K135 is above the 100mA level.
The P-channel complementary J49 (J50) has zero tempco at a lower level, ~75mA.
Biasing the P- and N-channel differently, plus have 0Vdc at the output, will be rather tricky.
Not all manufacturers provide these parameters or curves showing the drain current in relation to Temperature.
I assume that "my" 10N16/10P16 is close to original Hitachi
(I could not find any information on their datasheet)
To bias the two outputdevices differently is, as you say, not possible. I would therefore go for a bias that provides 100-125 mA through the soursresistors.
I started my attempt to change the voltage drop across the sourceresitors by changing R23. I went from 0.7 mV (330 ohm) to 1.5 mV (1360 ohms).
Since the changes gave so little effect, should I rather change the R20??
Eivind Stillingen
But I get a little bit confused. In one post it is referred to R20 (270 ohm) beeing the biasresistor, in other posts it is R23.
Is it to say that both resistors affect the bias?
Many of you have answered my question on "optimal" voltage drop across soursresistors.
The answers vary from 50 mA to 320mA.
Jacco wrote: Read the datasheets, zero tempco for the K134/K135 is above the 100mA level.
The P-channel complementary J49 (J50) has zero tempco at a lower level, ~75mA.
Biasing the P- and N-channel differently, plus have 0Vdc at the output, will be rather tricky.
Not all manufacturers provide these parameters or curves showing the drain current in relation to Temperature.
I assume that "my" 10N16/10P16 is close to original Hitachi
(I could not find any information on their datasheet)
To bias the two outputdevices differently is, as you say, not possible. I would therefore go for a bias that provides 100-125 mA through the soursresistors.
I started my attempt to change the voltage drop across the sourceresitors by changing R23. I went from 0.7 mV (330 ohm) to 1.5 mV (1360 ohms).
Since the changes gave so little effect, should I rather change the R20??
Eivind Stillingen
The optimal value is what sounds best to your ear with in the safe operating area of the device and having the required heatsinking to dissipate the heat.
Try 50mA, 100mA, 200mA and have a listen at each setting.
Regarding biasing do you have a trim pot to increase the bias or are you using fixed resistors?
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Have a look at post #563 from Keantoken. R20 sets the bias for the Mosfets.
Hope this helps.
Go read this article about biasing of L-MOSFETS, page 3 http://www.national.com/an/AN/AN-1645.pdf#page=1
Tommy/Keantoken
Clear answer and what I needed. Thank you. To morrow I will remount R23 to 330 ohm and use a pot for R20. I will go for 100mA as a start, then I will have a look at the scope.
It is a pity that the PCB dont have a trimpot for the bias from the start. For those who want, it should be easy to to change to a fix resistor later on.
Eivind Stillingen
Clear answer and what I needed. Thank you. To morrow I will remount R23 to 330 ohm and use a pot for R20. I will go for 100mA as a start, then I will have a look at the scope.
It is a pity that the PCB dont have a trimpot for the bias from the start. For those who want, it should be easy to to change to a fix resistor later on.
Eivind Stillingen
Nagys did not want a trimpot there because the Goldmund did not have one. A lot of people suggested that a trimpot would be a good idea in this place but you know Nagys stubbornness...
This is what I have "learned" today about my Goldmund clone.
Some of this adwises are perhaps "baby food"and "to simpel" for some of you, but for some inexperienced it may be okay with some practical advice.
There is is no problem to set the bias by using a potentiometer. I desoldered and liftet one leg of R20, let the resistance of 330 ohms to sit as a form of security. Between the free leg of this resistance and the "empty" eye on the PCB, I mounted a multiturn pot value 1 Kohm. A multiturn makes it possible to increase the bias gradually, and when the adjustment is completed, give a fairly accurate indication of how big the final fixed resistor should be.
I have so far landed at 135mA on each source resistance.
Setting the bias influence on the serieresistors R31/32.
R31/32 is placed on the PCB where there is little space, and where it is difficult to solder without having to turn the PCB (and dismantle PCB from the heat sink). It may be wise to think about this during soldering and before you do all settings/testing.
This will "easy" the installation of R31/32 and make it possible to solder from the component side of printed:
Create a "serie value "of R31/32 with two resistors. Solder these resistors standing and connect them with each other. When you have to change the total resistans value, change one of them, which you now do easily from the compont side.
With me, drivers T11/12 get hot. To hot, I think. First I choosed to use
MPSA 42/92 because I had these on hand. The drivers need, in my experience, cooling, or they will after some time "retiring" because of the heat. MPSA 42/92 does not have cooling possibilities.
The recommended drivers SS71/74 can be mounted with small heat sink.
I recommend to use those for drivers. They fit of course on the PCB.
Although, I chose to use some 150 V types. They were on hand. These are types of TO220 (BCE), and of course, fits very poorly into PCB, but with a little patience, this can be fixed. I expect that the drivers I use are of very little importance for listening result.
Work will continue to morrow.
Eivind Stillingen
Some of this adwises are perhaps "baby food"and "to simpel" for some of you, but for some inexperienced it may be okay with some practical advice.
There is is no problem to set the bias by using a potentiometer. I desoldered and liftet one leg of R20, let the resistance of 330 ohms to sit as a form of security. Between the free leg of this resistance and the "empty" eye on the PCB, I mounted a multiturn pot value 1 Kohm. A multiturn makes it possible to increase the bias gradually, and when the adjustment is completed, give a fairly accurate indication of how big the final fixed resistor should be.
I have so far landed at 135mA on each source resistance.
Setting the bias influence on the serieresistors R31/32.
R31/32 is placed on the PCB where there is little space, and where it is difficult to solder without having to turn the PCB (and dismantle PCB from the heat sink). It may be wise to think about this during soldering and before you do all settings/testing.
This will "easy" the installation of R31/32 and make it possible to solder from the component side of printed:
Create a "serie value "of R31/32 with two resistors. Solder these resistors standing and connect them with each other. When you have to change the total resistans value, change one of them, which you now do easily from the compont side.
With me, drivers T11/12 get hot. To hot, I think. First I choosed to use
MPSA 42/92 because I had these on hand. The drivers need, in my experience, cooling, or they will after some time "retiring" because of the heat. MPSA 42/92 does not have cooling possibilities.
The recommended drivers SS71/74 can be mounted with small heat sink.
I recommend to use those for drivers. They fit of course on the PCB.
Although, I chose to use some 150 V types. They were on hand. These are types of TO220 (BCE), and of course, fits very poorly into PCB, but with a little patience, this can be fixed. I expect that the drivers I use are of very little importance for listening result.
Work will continue to morrow.
Eivind Stillingen
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Yes, T11 and T12 definitely must be heatsinked. The heatsink that is proposed to use / Goldmund used is too small in size.
And yes, the transistors you chose are not suited for the job at all.
150V types are too weak.
Good luck
Lee Knatta
Yesterday I wrote incorrectly that the drivers MPSA 42/92 was replaced with 160 Volt types, to which you had objections.
2SA968A , which I have used is 180 V types and 2SC2238B is 200V. The voltage on the emitter on the T11/12 (and with a source current of 145mA) is now at the desired 80 V.
Do you still think I'm not on the "secure" side? Do you have suggestions for other types of drivers?
Time to day has gone into getting the voltages on the emitter to the series regulators T21/22 i"correct" and the desired flow through the sourceresistors. Results, look above.
I have also been listening, using some small Ursher speakers. The Norwegian importer of this speaker happens to be there (Roar do repairs for him). He was quite impressed with the amplifier's performance.
I have not yet had time to carry out measurements. Roar has promised to help with it early next week.
I hear absolutely no noise or hum in the speakers from the amplifier. It will be interesting to see whether measurements confirms what I have heard so far.
When it comes to "earthing", I have not done anything special:
0 volt from transformers from goes directly to GND on PCB board, speaker GND goes to the terminal on the amplifier's back panel. Input signal on the PCB card is recognized with unshielded (but twisted) wires to the input jack. Here at the input, earth is connected to chassis.
As I said I am waiting for a final jugde until measurements are fullfiled and more listening test on good(and large) speakers are done.
Both Roar and myself find this Goldmund very promising.
Eivind Stillingen
Yesterday I wrote incorrectly that the drivers MPSA 42/92 was replaced with 160 Volt types, to which you had objections.
2SA968A , which I have used is 180 V types and 2SC2238B is 200V. The voltage on the emitter on the T11/12 (and with a source current of 145mA) is now at the desired 80 V.
Do you still think I'm not on the "secure" side? Do you have suggestions for other types of drivers?
Time to day has gone into getting the voltages on the emitter to the series regulators T21/22 i"correct" and the desired flow through the sourceresistors. Results, look above.
I have also been listening, using some small Ursher speakers. The Norwegian importer of this speaker happens to be there (Roar do repairs for him). He was quite impressed with the amplifier's performance.
I have not yet had time to carry out measurements. Roar has promised to help with it early next week.
I hear absolutely no noise or hum in the speakers from the amplifier. It will be interesting to see whether measurements confirms what I have heard so far.
When it comes to "earthing", I have not done anything special:
0 volt from transformers from goes directly to GND on PCB board, speaker GND goes to the terminal on the amplifier's back panel. Input signal on the PCB card is recognized with unshielded (but twisted) wires to the input jack. Here at the input, earth is connected to chassis.
As I said I am waiting for a final jugde until measurements are fullfiled and more listening test on good(and large) speakers are done.
Both Roar and myself find this Goldmund very promising.
Eivind Stillingen
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I hear absolutely no noise or hum in the speakers from the amplifier. It will be interesting to see whether measurements confirms what I have heard so far.
Eivind Stillingen[/QUOTE]
In simulation the the schematic on page one has an output noise of 166 nV/Hz between 1 kHz and 200 kHz with a falling off slope above this.
I think this is quiet..
Eivind Stillingen[/QUOTE]
In simulation the the schematic on page one has an output noise of 166 nV/Hz between 1 kHz and 200 kHz with a falling off slope above this.
I think this is quiet..