What I meant by point me in the right direction, is that you tell me what to tell you. 
I want to tell you more but I don't know what else to tell you or to measure.
As far as I can tell, this isn't related to the previous problem. But if you want to know what the previous problem is, the DC at output drifts up and down over time. In the end it was found that the problem went away when the JFET stage is disconnected. So I ordered another pair of JFETs. It took a while to reach and when it did I kinda neglected the project. When I got back to it, the problem went away so I didn't replace the JFETs. Don't ask me why.
I'll post pictures later.
I want to tell you more but I don't know what else to tell you or to measure.As far as I can tell, this isn't related to the previous problem. But if you want to know what the previous problem is, the DC at output drifts up and down over time. In the end it was found that the problem went away when the JFET stage is disconnected. So I ordered another pair of JFETs. It took a while to reach and when it did I kinda neglected the project. When I got back to it, the problem went away so I didn't replace the JFETs. Don't ask me why.
I'll post pictures later.
Thanks.
The preamp is AMB's Beta 22.
The β22 Stereo Amplifier
It can drive headphones, or even efficient speakers directly. So drive capability shoudn't be a problem. Gain is set at 8x. Source is a typical wall powered DAC, I have not measured the output but I imagine it to be the standard 2Vrms. So it should put out 16Vrms. I have the volume set to more than 3/4 but it is soft and crackles and distorts.
Have you listened to headphones through your beta22? Just to make sure it's not the problem?
Regardless, the beta22 has lots and lots of gain and swing, so should be a good match for the F4.
Your Jfet are most very likely genuine...
I'm still thinking that you have a bad solder joint somewhere.
Regardless, the beta22 has lots and lots of gain and swing, so should be a good match for the F4.
Your Jfet are most very likely genuine...
I'm still thinking that you have a bad solder joint somewhere.
R9 is indeed 6k8.
I went back to my “working” channel and powered it up. It is biased cold at 150mV. The voltage across the TL431 as measured in the attached picture is 21 V. Detaching the PS, the resistance across the TL431 is 16.5K.
Tested the problem board in the same setup as above (see pic), the resistance is 21k (responds to P1 adjustment and kept at highest value). Powering it up, the voltage across the TL431 is 21 V. Now there is no measurable voltage across the power resistors in response to P1 adjustment.
I am tempted to “uncripple” it out of desperation. Either that or put it aside for a bit before I do something really stupid.
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Hi RonD2,
In the second pic, I can see a link in the C1 position, this is not helping if you have fitted a shorting link to the input.
If you do not build the crippled F4 to ZM's recommendations, you will have problems; for the moment, just cut the C1 link, and remove the input shorting link, does this return thing to 'normal' ie. same as the working channel?
In the second pic, I can see a link in the C1 position, this is not helping if you have fitted a shorting link to the input.
If you do not build the crippled F4 to ZM's recommendations, you will have problems; for the moment, just cut the C1 link, and remove the input shorting link, does this return thing to 'normal' ie. same as the working channel?
Do you have another power amp you could try?
First replace the F4, if everything sounds good, you have eliminated the preamp.
Next, there is a simple way to eliminate the JFETs by turning the F4 into a B1; connect the preamp outputs to the F4 inputs, then connect patch leads at the junctions of R3 & R4 and ground wires to the inputs 0V, then connect the (twisted) leads to the inputs of your other power amp; does it sound soft and distorted?
I “uncrippled” both of my F4 boards and have biased both at 120mV cold.
Now that Itsmee has taught me how to measure the TL431, the voltage reading concerns me. Both board’s TL431s measure about 12K R prior to power up. Both boards also measure 16.5V across the TL431 on power up at the 120mV bias. I have not let either warm up.
I have kept the 6.8K R9 resistors.
Should the 16.5V concern me?
Now that Itsmee has taught me how to measure the TL431, the voltage reading concerns me. Both board’s TL431s measure about 12K R prior to power up. Both boards also measure 16.5V across the TL431 on power up at the 120mV bias. I have not let either warm up.
I have kept the 6.8K R9 resistors.
Should the 16.5V concern me?
