I regret that I am not going to be able to diagnose your problem without being there, especially as you seem to know what you are doing. My guess is that it is one of the following:
1) Something at your end, probably cabling or ground related, or maybe a peculiarity of your amp. This is most likely, as the nature of PRE/POST is that if almost anything goes wrong, you get mains hum or feedback.
2) A hardware problem in the unit. I’ve not heard of anything that will do as you describe, but there is always a first. If you think that this is the case you should send it in for service ( support@eventide.com or your dealer ).
3) It could be a software problem, but none of the many many people using this feature have reported it.
I’ll give you more infos:
I tried to use the pre/post without the FX loop of the amp. It means something like: guitar-H9Pre-effect pedals-H9Post–Amp input … and it works great (using Hi gain distortion pedal too!).
Next thing that I’ll do is going to a friend of mine and try with another amp with different Fx loop.
By the way, I’ve noticed a peculiarity on the feedback/hum that I hear: in the Fx loop set on Post, the bad sounding delays (or any other fx) got a metallic “flanger” sound… it’s like a faulty EHX Electric Mistress attached to it.
I’ll post here what Koch writes on the Fx loop:
” A good Effects Loop has to include an electronic circuit that:
1) brings the high signal level of the tube amp back to the low signal level of the effects unit which goes to the Send input on the amplifier
2) brings the low signal level of the Return jack back to the high signal level of the tube amp
3) matches the impedances of the tube amp to those of the effects unit.
4) does not affect the sound of the amp (!!!)
….. All our Effect Loops operate on a signal level of -10dBV which perfectly matches the signal level of all battery operated effects units. I always use a “buffer” circuit to drive the Send output. This buffer circuit brings the high impedance of the tube amp down to a very low output impedance. Therefore, our Send outputs will drive any effects unit on the market with minimum signal loss due to cable influences. “
So…since the problem appears only in the Fx loop I think the problem lies in some ground problems…uff. If, on the other hand, the problem is hardware but it appears only when I use the H9 both in input and Fxloop, the problem must lie in something that checks the signal level and changes some parameters…e.g. :from the link I read:”Warning: Set up your H9 for Pre/Post routing mode (“Changing routing modes” below) before making cable connections. Otherwise you risk feedback or clipping of your signal.”
…does the pedal adapt to the tension like it does for the expression pedal?
and
“Note: When hooking up your gear in this manner, you may encounter a ground loop which could introduce a low frequency hum in your signal. This is not a fault of the H9 or your other gear, it is a grounding issue. Please see the EventideAudio.com website for a discussion and tips on ground loops.”
