Loss of highs plugging into effects return/power amp in. help!

[taha_never]

when i plug my external preamp right into the effects return on my rumble 60 v.1 i lose a huge amount of highs even with the ultra Hi engaged on the pre. is it normal?
setup: passive P > tube preamp (i have no specs on this unit except it has a 1/4 mono output) > rumble 60's return.
cheers

Edited January 29, 2016 by Yawn_Blah

[dincz]

Effects returns are generally lower impedance than instrument inputs. Not a problem for most preamps but maybe yours has a fairly high impedance output.

[Bill Fitzmaurice]

[quote name='dincz' timestamp='1454011816' post='2965477']
Effects returns are generally lower impedance than instrument inputs. Not a problem for most preamps but maybe yours has a fairly high impedance output.
[/quote]+1, it's probably a loading issue related to the output impedance of the pre.

[taha_never]

thanks a lot now i have a picture of what's happening. search results too technical for a noob so i need you guys to confirm that i can ask a tech to reduce the output impedance on the pre?

[Beer of the Bass]

Can you tell us much about the preamp, even just which valves it uses and what controls it has? Depending on the circuit it might be necessary to add an extra active stage to the preamp to lower the output impedance. A good tech would be able to do this with a high voltage FET if the power supply and chassis space can't support another valve, similar to the circuits often used to drive effects loops.

[taha_never]

thanks Beer of the Bass, here is a picture of what's going on inside. 12ax7 are the tubes. normal and brite inputs, gain, U-hi U-lo, bass, trbl and master are the controls.
[sharedmedia=core:attachments:201501]

Edited January 29, 2016 by Yawn_Blah

[taha_never]

i may prefer the FET over another valve, though there is enough room for a chandelier inside the housing!

[Bill Fitzmaurice]

[quote name='Yawn_Blah' timestamp='1454049351' post='2965723']
thanks a lot now i have a picture of what's happening. search results too technical for a noob so i need you guys to confirm that i can ask a tech to reduce the output impedance on the pre?
[/quote]That's not something you can do with a screwdriver. I don't see an output transformer, so my guess is that it was designed to operate like a pedal into the bass amp input, rather than driving a power amp. One would have to see the specs to be sure, but if it works OK that way it's a safe assumption that's the case.

[dincz]

Looks like there's an op-amp on the output board so I'd suspect it's a buffer stage intended to drive low impedances. It could be faulty or badly designed.

[taha_never]

yes Bill thanks for the consultation, it sounds ok into the amp's input but not into the power amp return.
thanks dincz for the input, yes that's an op-amp which were suspected by other chaps on another thread about it causing a lot of white noise.

so what is the solution to have a suitable output to drive the power amp on my rumble 60 (18k ohm return) without a problem?

[Bill Fitzmaurice]

Without a schematic one can't be sure. Typically you want the output impedance of a line driver to be no more than 1/10 the impedance of the driven input.

[taha_never]

[quote name='Bill Fitzmaurice' timestamp='1454085584' post='2966346']
Typically you want the output impedance of a line driver to be no more than 1/10 the impedance of the driven input.
[/quote]
that'd be the key. thanks a lot

[dincz]

It's probably time for a schematic and/or a tech.

[taha_never]

[quote name='dincz' timestamp='1454086409' post='2966356']
It's probably time for a schematic and/or a tech.
[/quote] i don't have the schematics for it and the builder is out of my reach. i have to look for a tech. but thanks for helping me with pointing out the problem source. Cheers

[taha_never]

does it make sense to use a DI between the pre and rumble's return? to balance things out?

Edited January 30, 2016 by Yawn_Blah

[Beer of the Bass]

[quote name='Yawn_Blah' timestamp='1454130940' post='2966764']
does it make sense to use a DI between the pre and rumble's return?
[/quote]

If you plug the preamp output into an active DI and use the XLR output via an XLR-jack cable, that may work. It would at least allow you to identify whether this is an impedance issue.

[taha_never]

[quote name='Beer of the Bass' timestamp='1454143551' post='2966802']
If you plug the preamp output into an active DI and use the XLR output via an XLR-jack cable, that may work. It would at least allow you to identify whether this is an impedance issue.
[/quote] so the passive ones are not effective for my situation? i'm having a good deal on a fender mini DI right now!

[Guest bassman7755]

If you've got to the point where your spending money on this I'd honestly consider taking a punt on something like the [url="http://www.juno.co.uk/products/behringer-bdi21-v-tone-bass-driver-di/493811-01//?currency=GBP&flt=1&gclid=CjwKEAiA27G1BRCEopST9M39gykSJADQyqAlZntZspi0GzuWd5i_X1VjhEJ6QT_wmPupTgcm9MxN0hoCdcvw_wcB"]Behringer DBI[/url] - £25 for something that will on its own probably give you as good (possibly even better) a sound as your preamp.

Edited January 30, 2016 by bassman7755

[taha_never]

tried the DI. it didn't solve this problem!