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Old 30th June 2004, 02:43 AM   #1
mourip is offline mourip  United States
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Default GC as HEadphone Amp...

Hi,

I am in the home stretch for putting together my GC amp based upon Brian's boards. Since I often use headphones for listening to music so as to ensure domestic tranquility, I am starting to think about wiring up the GC with a headphone jack. This is not new turf for me as I have built several homebrew headamps using op amps, a SS buffer, and a couple of tube designs. my last two amps were tubed and I essentially just drove the phones straight off of the highest impedence tap on my output trannie. This has worked well.

What got me intersted in the GC was the rave review on 6 Moons regarding using the GC as a headamp to drive some AKG 1000's. I am not sure if I will spring for the AKG's but I will use my Beyerdynamic 990's which are 250 ohm. Very easy to drive.

For those of you who have stayed with me here is my question. How important is it to use impedance matching resistors on the output for driving phones? I figure that with the amp rated at 4 to 8 ohms I should be fine since the output impedance of the amp is lower than the nominal impedance of the phones.

Any opinions out there?

Best,

Paul
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Old 30th June 2004, 05:55 AM   #2
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You don't have to load the GC with 4 or 8 ohms, higher impedance is perfectly OK.
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Old 31st January 2005, 12:12 PM   #3
breez is offline breez  Finland
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How should I go about if I want the gc's output down to proper levels for headphones? This in the case where the amp will have a gain of 20 and needs to drive speakers when not driving headphones. Voltage divider or some other resistor mess at the output doesn't seem like a good thing, high output impedance seen by the phones is a big as the response curve will be mangled.

My original option is still a simple separate opamp circuit (cmoy style). Any ideas on how to switch between speakers and phones? Would prefer no switches (other than the rotary switch attenuator) on the signal path if possible. Any downsides if I have the inputs connected to both opamps (speaker amp and headphone amp) all the times and then just control which circuit gets power?

Thanks,
Lauri
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Old 31st January 2005, 12:34 PM   #4
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Probably you have seen it, but anyway, here is my suggestion. Build the 'high end' preamplifier (OPA 627 + BUF634) from CarlosFM, maybe give it a bit more gain so it has sufficient power for the headphones. You can make a line out, with a smaller output cap, for the gainclone and a headphone out, with a larger output cap, for the headphone.
High-end preamp for my GC

This way you have a headphone amplifier and you can test if you like the gainclone more with a preamp or just a (passive) attenuator.
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Old 31st January 2005, 12:36 PM   #5
mourip is offline mourip  United States
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Default GC as Headamp

Hi Lauri,

I have been using my GC with 250ohm Beyerdynamic phones for several months. I am very happy with the results. I have looked often throught the various web forums for information regarding impedance matching and mostly found theories. My own listening experience is that running headphones directly from the output of a regular amp can produce fine results. I have used both tube and SS amps with good results.

On my GC I made two concessions for phones. First I have a switch for phone/off/speakers. Second I have a high quality resistive voltage divider on the input which allows my volume control to work in a reasonable range. This will decrease the maximum volume for speakers but since I mainly use headphones this is not a problem for me. You could probably switch the divider in and out of the circuit if you wanted, either on the output of your preamp or the input of your amp. Since GC's are cheap to make it makes sense to consider using it for dedicated HP use.

I think that headphilers are missing many opportunities for good sound by not experimenting with full size amps. My current project is rebuilding a Sonic Impact amp for headphone use. The only gotcha there is that you cannot tie the two speaker grounds together which means rewiring the headphone cable.

So many projects, so little time.

Best,

Paul
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Old 31st January 2005, 01:08 PM   #6
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You must to do on output of amp voltage divider cca 4 : 1, with output impedance cca 4 Ohm ( 22 Ohm, 10-15 W/ 4.7 Ohm, 2 W ), 'cos 990 as dynamic transducer must be driven from low impedance source for minimal additive ripple of frequency characteristic.
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