Cool simple "clean" CFA

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
vzaichenko

vzaichenko

R.I.P.
Joined 2010
still4given said:
Hi Valery,

I changed out R1 & R3 to 27R. Very little change.

I tried 47R for R6 & R7 and that brought it down a little.

I now have 68R there. For some reason the two channels are different. I'm guessing difference in gain of the various transistors but not sure. Here are the readings I'm getting now.

Left channel
R2 (1k5) = 5.619V = 374uA
R4 (1k5) = 5.507V = 366uA
R6 (68) = 1.311V = 19mA
R7 (68) = .922V = 13.59mA

Right channel
R2 (1k5) = 6.0V = 4mA
R4 (1k5) = 5.425V = 3.61mA
R6 (68) = 1.797V = 26.4mA
R7 (68) = 1.656V = 24.35mA

May I assume this IPS was developed with the Tubsumo OPS in mind? Is it possise this IPS with the Slewmaster? I think the IPS needs to be around 5mA.

Thanks, Terry
Click to expand...

Hi Terry,

You are right, I had Tubsumo OPS in mind here. However, I have tested this front-end with both Tubsumo and IRFP version of Slewmaster - the range of the bias pot was enough to adapt to the higher VAS current.

It is really strange for me that you've got so much of a difference between the left and right channels.

Also,

R2 (1k5) = 5.619V = 374uA
R4 (1k5) = 5.507V = 366uA

are not really correct calculations, should be like this:

R2 (1k5) = 5.619V = 3.75mA
R4 (1k5) = 5.507V = 3.67mA

What could be the reason for such a big difference... transistors' parameters tolerance? I did not match the FETs, but my output currents are pretty close to each other in both channels.

Cheers,
Valery
 
still4given

still4given

Member
Joined 2004
Sorry for the wrong calculations. I must have entered the data wrong.

I know that the NFB doesn't work properly without the OPS attached but the offset is pretty high like this. Coule trimmers be put in place of R6 and R7 for more flexability?

I'll try it with the Tubsumo OPS first but I would rather it worked with the Slewmaster since I have more of them.

I did match the input pair but not channel to channel. I try hooking it up to the OPS today and see what I find.

Thanks, Terry
 
still4given

still4given

Member
Joined 2004
Hi Valery,

I hooked it up to the Tubsumo OPS. I have almost 4V offset. I am looking closely to the pinout of the servo and it looks wrong to me. Can you please have a close look at it and tell me if I'm right? I'm thinking the + leg needs to go to pin 7 and I believe it is going to pin 8. If this is wrong, could that be the reason things are so off? With the servo removed I still have about 2V offset.

Thanks, Terry
 

Attachments

  • CF-FET Gray.jpg
    CF-FET Gray.jpg
    104.1 KB · Views: 269
still4given

still4given

Member
Joined 2004
Here's an update. I was pretty sure about the servo so I just bridged across pin 8 and pin 7. That worked fine on the right channel. It drops to about 12mv offset after a few seconds. However, the left channel only drops to about .9v offset. I tried pulling both servos and the right channel has a about .9v offset and the left channel is at about 2.1v. Another thing I noticed is that Pin 7 on the left channel servo drops from 14.5V to about 5.6v over about 10 seconds from power on. I tried a few different ICs and they all do the same. Any idea what might cause this? One more thing of note is that with the servos pulled I have +-14.5V on the right channel zeners but on the left I have -15.3 and +14.5. I doubt this is causing that voltage drop but it is interesting. Any ideas are welcome.

Blessings, Terry
 
T

thimios

Member
Joined 2009
still4given said:
Hi Valery,

I hooked it up to the Tubsumo OPS. I have almost 4V offset. I am looking closely to the pinout of the servo and it looks wrong to me. Can you please have a close look at it and tell me if I'm right? I'm thinking the + leg needs to go to pin 7 and I believe it is going to pin 8. If this is wrong, could that be the reason things are so off? With the servo removed I still have about 2V offset.

Thanks, Terry
Click to expand...
Hi Terry,according to what schematic is this IPS?
If it is from post 248 ....yes it is wrong pcb connection.
 
Last edited:
still4given

still4given

Member
Joined 2004
thimios said:
Hi Terry,according to what schematic is this IPS?
Click to expand...

Hi Thimios,


This one has been a little tough because the schematic for this layout is in post 229 but Valery changed the part numbers as he continued development so the most recent schematic has different part numbers than this board.

Hi Jeff,

I'll try matching a pair of 1m resistors and replace the ones in the servo circuit and see if that makes any difference. Any ideal what is pulling down the voltage on the +rail for the servo?

Thanks, Terry
 
J

jwilhelm

Member
Joined 2014
vzaudio.com/
Is your voltage drop the same on R38 and R39? Looking at the circuit, the only thing that could cause uneven voltage would be the op amp or the zeners themself. Maybe try popping the op amp out if it's in a socket and see if the voltage gets better. If not, there is a problem with the zener or a copper whisker somewhere.
 
Last edited:
still4given

still4given

Member
Joined 2004
Hi Jeff,

If I lift R24 pin 6 goes to 13.44v and pin 7 holds at 14.46V.

Hi Thimios,

I'm not sure there are other layout problems because one channel seems to be working. I think I must have a bad part somewhere but I can't seem to find it. All parts read the same from channel to channel with and ohm reading or diode test. It is only under power that I am seeing these issues.

Thanks, Terry
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom