There's about a 3dB difference in 3rd harmonic at the higher bias, but the 2nd seems to rise at 2A.
I find the best setting is 1.6A for this build with three pairs. This is at -18dB from my source, which is about 4V RMS into a 5 ohm resistor (because I happen to only have 10 ohm resistors big enough to handle the test).
I have seen lower numbers for 2nd and 3rd, but rarely have I seen only the second and third with all other harmonics under noise floor.
The subjective impact of the sound is not too different between 1.6 and 2A. I'm sorry I haven't tried 1A bias.
The temperatures in our city were around 14 degrees and I routinely measured 20-22 degrees in room. If your airconditioner setting is close to that, the sinks can take 2A. For a more realistic room temperature of 27 degrees, 1.6A is a better bet and what the amp will finally be biased at. If you have been following Rixsta's pictures the heatsink are 3x 7" tall pieces of what we term the 'flower' heatsink (an unfortunate name, but we'll stay with it) per channel.
I find the best setting is 1.6A for this build with three pairs. This is at -18dB from my source, which is about 4V RMS into a 5 ohm resistor (because I happen to only have 10 ohm resistors big enough to handle the test).
I have seen lower numbers for 2nd and 3rd, but rarely have I seen only the second and third with all other harmonics under noise floor.
The subjective impact of the sound is not too different between 1.6 and 2A. I'm sorry I haven't tried 1A bias.
The temperatures in our city were around 14 degrees and I routinely measured 20-22 degrees in room. If your airconditioner setting is close to that, the sinks can take 2A. For a more realistic room temperature of 27 degrees, 1.6A is a better bet and what the amp will finally be biased at. If you have been following Rixsta's pictures the heatsink are 3x 7" tall pieces of what we term the 'flower' heatsink (an unfortunate name, but we'll stay with it) per channel.
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Couple things that I would like to get clarification.
What is your temp on heat sinks after 1 hour run?
I have E type sink (55cm x 22cmx 8cm) and 4 pairs (Toshiba 2SK1530/2SJ201) per mono-block. With 1A, I'm reaching 55 deg C on sink in 40 min and then it's stays constant. I do not think I can go higher with current since I might exceed safety acceptable limit. May be make send to installed fans, but I did not consider that option yet.
About harmonics for P3 adjustment, I use my distortion analyzer and minimized the 2nd one to its min. I assume 5 and 10 ohm that you mentioned were related to the load. I used 4R 10W as Nelson advised and 2.8V (if I remember correctly) for output.
My problem is that I'm trying to get 32VDC out of my supplies, but in reality I see 28-28.5VDC out under load and both units are connected to power. I thought that Schottky Rectifiers with very low Vf might resolve my case and I'll see 30+VDC there.
After one hour the temperatures on the sinks are about 45 degrees. Note the low ambient as of this writing, and 1.6A bias setting.
It takes about two hours for them to reach 49 degrees where they stabilise. Add five degrees to ambient and we will be around 55. In reality the thermistor begins to kick in and limit the bias to keep the temperatures around 52 degrees. At this limiting point, the last time I checked the bias had settled to 1.6A. At the present moment bias stays high, around 1.75A. Also my sinks are in the open. Once they get fitted to a case and the lid is closed, the bias can come down a bit.
I will try and post a couple graphs of bias and temperature versus time. I have a feeling if you are running out of sink at 58w per side, you need to reconsider whether the F5T is the correct amplifier for your sink. Pushing the rail higher than what it is will cause your bias to be further dropped to maintain a good temperature.
That being said, the Pass XA 30.8 was tested to reach sink temperatures of 60c. I have also encountered fan failure on one build that had it, causing the sink temperature to hit 90 degrees, and it was in the middle of a listening session so we never realised till it was time to turn the amp off. It still played fine and worked well after, suggesting that there is quite a bit of safety margin in the design and the devices.
It takes about two hours for them to reach 49 degrees where they stabilise. Add five degrees to ambient and we will be around 55. In reality the thermistor begins to kick in and limit the bias to keep the temperatures around 52 degrees. At this limiting point, the last time I checked the bias had settled to 1.6A. At the present moment bias stays high, around 1.75A. Also my sinks are in the open. Once they get fitted to a case and the lid is closed, the bias can come down a bit.
I will try and post a couple graphs of bias and temperature versus time. I have a feeling if you are running out of sink at 58w per side, you need to reconsider whether the F5T is the correct amplifier for your sink. Pushing the rail higher than what it is will cause your bias to be further dropped to maintain a good temperature.
That being said, the Pass XA 30.8 was tested to reach sink temperatures of 60c. I have also encountered fan failure on one build that had it, causing the sink temperature to hit 90 degrees, and it was in the middle of a listening session so we never realised till it was time to turn the amp off. It still played fine and worked well after, suggesting that there is quite a bit of safety margin in the design and the devices.
Alex please confirm what is your measured transformer secondary and PSU voltage under load with both amps on? Are you using a CRC setup as described in the F5T articles?
Nash
I'll measure that during this weekend. Yes, I use all available spaces with advised value of resistors as CRC. The question is if to replace one or more spots of resistor with 1mH air coil as Spenser using in his builds?
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you say you have 2x24VAC transformers. under which condition?
at what main voltage and under which load should the transformers give you 2x24VAC? if this is with 120V mains under no load it makes sense that your voltage is low.
have you measured the actually voltage output of the transformers under load? i guess it's around 2x21VAC with the main voltage you got.
at what main voltage and under which load should the transformers give you 2x24VAC? if this is with 120V mains under no load it makes sense that your voltage is low.
have you measured the actually voltage output of the transformers under load? i guess it's around 2x21VAC with the main voltage you got.
Very interesting info and as you see my latest correspondence, I assumed 1.6A per MOSFET. So, I have 4A Bias.
I missed the 470mV post - that is what happens when I reply to email late in the night.
I'm not surprised your rail voltage is sagging. a 900VA transformer would not be happy at that level of constant current.
Your heatsinks OTOH seem to be up to quite a bit of beating. You could reduce the bias a bit/see what kind of THD numbers you get. Doubtless it will also relax the load on your transformer and get the rail voltage a bit higher.
The Toshiba devices have slightly different characteristics than the standard IRFP devices I use, so your results may not be directly comparable. I expect them to be quite a bit better actually.
I'm not surprised your rail voltage is sagging. a 900VA transformer would not be happy at that level of constant current.
Your heatsinks OTOH seem to be up to quite a bit of beating. You could reduce the bias a bit/see what kind of THD numbers you get. Doubtless it will also relax the load on your transformer and get the rail voltage a bit higher.
The Toshiba devices have slightly different characteristics than the standard IRFP devices I use, so your results may not be directly comparable. I expect them to be quite a bit better actually.