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Old 18th December 2009, 03:16 AM   #1
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Default trouble matching line level to guitar amp input level...any help?

hi folks.

Ive posted in the musical section, but thought it may be worth me posting here.

I have a valve guitar amp, as well as a pedal effects box also using a valve within it. seperately they both sound amazing but together they sound awful.

I am a qualified electrical eng but i have to say that valves ceased to be mainstream HIFI before i was born so i know little about them. I belive the issue im having is one of impedance-matching the headphone/line level output to the guitar(high impedance?) input of the amp.

anyone have any advice out there?

also, can anyone tell me how to gauge whether i need to replace my output valves(or indeed any of the preamp ones either)


**before anyone says it...i work with 33Kv on a daily basis, so i understand safe c.o.d's and have no qualms with SAFELY tinkering around inside my powered up amp-only if necessary**

Last edited by mondogenerator; 18th December 2009 at 03:21 AM.
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Old 18th December 2009, 03:41 AM   #2
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Quote:
Originally Posted by mondogenerator View Post
I belive the issue im having is one of impedance-matching the headphone/line level output to the guitar(high impedance?) input of the amp

anyone have any advice out there?
What does "awful" mean? No bass? No treble? Distortion? Generally, a line or headphone output should be reasonably low Z, and the input to the amp should be high Z, which is just what you want. So, knowing what awful means, and perhaps knowing what the effects box and amp are, would be helpful.

Quote:
also, can anyone tell me how to gauge whether i need to replace my output valves(or indeed any of the preamp ones either)
Use a tester. Or, better yet, hook up the tube in standard grounded cathode configuration and measure whether it meets specs. If you don't know how to do that, take a look here: Neets Module 06-Introduction to Electronic Emission, Tubes, and Power Supplies With your background, it should take you all of 15 minutes to be up and running.
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Old 18th December 2009, 04:56 AM   #3
cbdb is offline cbdb  Canada
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Thats unusual. The pedal output should be low impedance and the amp in high, which shouldnt be a problem. A guitar straight in to a line level input is another thing and in the studio you use a DI box (direct input ) to match impedances. Does the pedal documentation state an output impedance? What does the pedal do? Is it just a matter of too much distortion from both pedal and amp?
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Old 18th December 2009, 09:41 AM   #4
Enzo is offline Enzo  United States
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My first thought is you are simply overdriving the guitar amp input. Line level signals are WAY hotter than guitar signals, and headphones outputs tend to be even hotter.

I would say the "valveness" of the systems is not the issue.
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Old 18th December 2009, 11:57 PM   #5
jjman is offline jjman  United States
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Yes, need to see a schematic of the effects unit, especially it's output. May just need to attenuate it's output.
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Old 19th December 2009, 10:00 AM   #6
m6tt is offline m6tt  United States
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Theories:

Impedance seen by pedal too high
DC from pedal upsets tube bias
Voltage out from pedal too high

Solutions:

Pull-up resistor in hot lead
Cap in hot lead
Volume control

Combined:
Volume control with cap between signal in and pot. Pot value sets input impedance in parallel with Rgk, likely 470k to 1M for most guitar amps. Use wiper for signal out. Cap value sets low frequency roll-off in conjunction with input impedance, select for tone.

Good luck, I'm sure it'll sound pretty great when it works!
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