What effect would having the bias off have?

PreCd

I have a GFA-555 that I have went through and replaced all caps, even the large filter caps. DC offset is very good sitting at 9mv per channel.

I would like to check the bias, have never done that, and was wondering what effect would it have on the sound if the bias were off?

heiney9

http://en.wikipedia.org/wiki/Biasing

http://www.tnt-audio.com/clinica/bias_e.html

Couple of good readings to expand on it a bit.

Correct bias allows the transistors to operate in their intended "sweet spot". The short answer is amp that has extreme bias issues will have more distortion and it's most likely to be audible. It can also sound lean and closed in. Over bias can cause the transistors to saturate and again audible distortion and in extreme cases, especially with bi-polar transistors they can go into thermal runaway and eventually burn out.

I'm sure Fred will correct me if I'm wrong and hopefully he will add more info.

H9

heiney9

It keeps the transistors in their linear range, giving the best possible sound.

The amp can sound hazy, closed in, lean, compressed, etc if the bias is not set properly. It is a good idea to check it.

H9

FTGV

PreCd wrote: »

I would like to check the bias, have never done that, and was wondering what effect would it have on the sound if the bias were off?

Since it's a push pull output stage crossover distortion will rear it's ugly head if bias is set less than optimum,likely giving it a hard / sterile flavour.If set too high while it may sound good will eventually lead to a failure of the output devices.I think I read in another thread that you have a service manual so it should indicate the proper level (measured in millivolts)and the correct test points at which to take the reading ie.such as across an emitter resistor.Be careful here as a slipped meter probe can cause a lot of grief in the form of blown unobtanium output tranasistors.

PreCd

FTGV wrote: »

Since it's a push pull output stage crossover distortion will rear it's ugly head if bias is set less than optimum,likely giving it a hard / sterile flavour.If set too high while it may sound good will eventually lead to a failure of the output devices.I think I read in another thread that you have a service manual so it should indicate the proper level (measured in millivolts)and the correct test points at which to take the reading ie.such as across an emitter resistor.Be careful here as a slipped meter probe can cause a lot of grief in the form of blown unobtanium output tranasistors.

I do have a service manual however it is not for my flavor of version 1 555 but a later version of the GFA-555 version 1. I am going to follow the warm up procedures in the manual but the designations are different in that manual from the board I have. Here is what I plan on doing, taking into consideration I follow the warmup/cool down procedures:

Place my VM on each side of the emitter resistor pictured below:

Then the adjustments will be made via the pots on each side of the board below allowing me to adjust for bias on the left and right channel.

I will repeat this procedure for each channel following the warmup/cool down procedures in the manual. Sound about right?

heiney9

If you go over to DIYAUDIO.com there are several long, detailed threads on the 555. It should give you a step by step with proper bias voltage. The 555 II is biased quite a bit higher because it has larger heat sinks and different output configuration, so I wouldn't trust that value for the series I amps.

H9

PreCd

heiney9 wrote: »

If you go over to DIYAUDIO.com there are several long, detailed threads on the 555. It should give you a step by step with proper bias voltage. The 555 II is biased quite a bit higher because it has larger heat sinks and different output configuration, so I wouldn't trust that value for the series I amps.

H9

I tell you it is a challenge to get where you want to go over there at DIYAudio. Great bunch but the threads get so long.

Thank you for your responses. The manual I have is for the GFA-555 Version 1 which is what I have but there are more than one flavor of that version 1. Just so happens I have the service manual for the other flavor which is not a great deal different but it is different. Here is the bias adjustment procedures for the GFA-555 Version 1 but a little different flavor than mine. I assume I can just run the amp a about 1/3rd volume to achieve the correct temp instead of the 66W into 8ohms:

PreCd

I got the amp all put together over the holidays. Changing those 26 year old filter capacitors significantly changed the sound. More musical. Less muddy. Greater detail.

The amp could use a bias checkup but I am having a terrible time finding documentation on adjusting it. I have a GFA-555 service manual but it contains the procedure for a PS board unlike mine. They may be similar enough that the procedure also pertains to it but I would like to run it by the forum. Hopefully some of the past Adcom techs will chime in or others who have the same board as mine.

Here is my board along with the link to the schematic:

Link to schematic: ftp://www.civilwarmedicalbooks.com/b...schematics.pdf

My GFA-555 PS board:

Here is the link to the only GFA-555 service manual I can find which has the bias alignment procedure in it. Remember the board in this manual differs from mine:

ftp://www.civilwarmedicalbooks.com/bias/Adcom_GFA-555_service_manual(1).pdf

Here is board the top link refers to:

PreCd

Got the links to the schematic and service manual screwed up in the previous post. Here are the corrected versions:

Here is my board along with the link to the schematic:

Link to schematic: www.civilwarmedicalbooks.com/bias/hfe_adcom_gfa-555_schematics.pdf

Here is the link to the only GFA-555 service manual I can find which has the bias alignment procedure in it. Remember the board in this manual differs from mine:

www.civilwarmedicalbooks.com/bias/Adcom_GFA-555_service_manual(1).pdf