The preamp has gone through a bunch of changes. It had hum issues that were hard to cure. I ended up designing a steel chassis with internal shielding for the power supplies. That's waiting for a test run. I added optional tone controls. I have a couple different options for volume controls waiting to be tested. I have a couple different output stages designed for it. I/O boards are modular with balanced and/or single ended options.
One big hurdle I have come up against is the output stage based on the VHex amp is an oscillator. I'm not sure if Valery actually did a full analysis on this output stage or just adapted it from the headphone amp and rolled with it but when I simulated it in LTSpice the phase margin wasn't good. Unfortunately the guy I always turned to in a situation like this isn't with us any more so that output stage design is on hold for a while.
I have a few other projects eating up all my spare time at this is on the back burner at the moment.
One big hurdle I have come up against is the output stage based on the VHex amp is an oscillator. I'm not sure if Valery actually did a full analysis on this output stage or just adapted it from the headphone amp and rolled with it but when I simulated it in LTSpice the phase margin wasn't good. Unfortunately the guy I always turned to in a situation like this isn't with us any more so that output stage design is on hold for a while.
I have a few other projects eating up all my spare time at this is on the back burner at the moment.
I had a plan to use VHex as a line-stage preamplifier. What could be the reason for oscillations?
Any chance that PCB tracks are crating problems?
Do you have any suggestions for alternate line stages?
The phase of the output is shifting about 170 degrees before the gain of the preamp drops to zero, up around 15 MHz if I recall correctly. This causes the feedback to switch from negative to positive so the amp goes into oscillation. I don't know enough about it (I normally work in the digital domain) but I think the bandwidth of the preamp just needs to be limited. If someone wants to try to come up with a fix for it I've attached my LTSpice files.
Valery's Valvolex input stage works great. It's a tube buffer but it requires a high voltage supply to operate. The VHex+ headphone amp would work as well, but it's a bit extreme for this purpose.
Valery's Valvolex input stage works great. It's a tube buffer but it requires a high voltage supply to operate. The VHex+ headphone amp would work as well, but it's a bit extreme for this purpose.
Hi guys,
I made an attempt to fix the phase and gain margins. I am attaching herewith the moded ltspice files.
I am not so familiar with preamps so the mods needs to be verified first. Also there seems to be a very slight shift in bias currents.
PM reads -113° and GM reads -14dB.
I made an attempt to fix the phase and gain margins. I am attaching herewith the moded ltspice files.
I am not so familiar with preamps so the mods needs to be verified first. Also there seems to be a very slight shift in bias currents.
PM reads -113° and GM reads -14dB.
Attachments
You are welcome!
Check the attached asc files. That is your asc file, I only changed some component values.
