Preamp question
May 30, 2002 at 11:05 PM Thread Starter Post #1 of 15

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Can you use a preamp w/out headphone output as a headphone amp? I am considering a Menthor Integra hybrid with a sweet pre stage. What I'm thinking is to connect the headphones to the rca out jacks in the back of the pre via an adapter.

any thoughts?

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May 30, 2002 at 11:39 PM Post #2 of 15

Audio Redneck

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Been there, done that, not pretty.
 
May 30, 2002 at 11:44 PM Post #3 of 15

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can you elaborate a little? what was the problem? What kind of pre did you use?
 
May 31, 2002 at 12:17 AM Post #4 of 15

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Tried it with my current pre (adcom GTP-400) with my V6s, and a pioneer and marantz back in college with the Koss cans I had (wish I remember the model, but they were about the '85 equivilant of Portapro's).

Results varied somewhat, but none were good - "tinny" was the common word. Had better luck comming straight out of the source with the cable, but that wasn't right either - bass just isn't there.

I'm going out to the garage and look for the adaptor now and will try again so I can give you a better description.
 
May 31, 2002 at 12:21 AM Post #5 of 15

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thanks AR
will look forward to your next post.

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May 31, 2002 at 4:59 AM Post #6 of 15

Dusty Chalk

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http://www.divertech.com/antiquesl.html

Look for "Reference Tools".

I want to try it, too, with a Musical Fidelity A3^CR...as soon as I get one...yes, Hirsch, I saw your email, it's tempting...
 
May 31, 2002 at 2:44 PM Post #7 of 15

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Quote:

Originally posted by 2 channel
thanks AR
will look forward to your next post.

2


Well, I hate to dissappoint you, but it will be a day or two. I've got two of those stupid things (1/8" & 1/4") and can't find either of them.
mad.gif
I'll be looking again tonight. (Time to clean the garage again - anyone want some junk?)
 
May 31, 2002 at 6:31 PM Post #8 of 15

sirion

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I'm in the same boat as you, 2 Channel...(And got referred to this thread when I posted here last night).

What puzzles me is that my preamp looks like it would work great; it's has a Class A amplifier, no opamps, etc...My main problem is that I'm very new to this whole high-quality headphone thing, so just plugging in some random headphones into it won't tell me anything, as my ear isn't trained yet.
 
May 31, 2002 at 7:47 PM Post #9 of 15

Hirsch

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The output of a line stage preamp is designed to operate into a power amplifier. Looking at the spec for the first power amp I could think of, the Berning ZH-270 has an input impedance of about 27K ohms. 47K ohms is more typical of a power amp. A high impedance headphone, such as Sennheiser, has an impedance of 300 ohms. Sony's have an impedance of 32 ohms. Do you begin to see why a headphone presents a very different load than the one a preamp is designed to drive?
 
May 31, 2002 at 7:52 PM Post #10 of 15

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That makes perfect sense now, thanks Hirsch.
Here is another question then,
I am thinking of moding my MG Head DT. I am thinking of eliminating the mini jack on the front and re-routing the wires to the back of the amp and terminating them w/ RCAs so I can use the MG head as a preamp...what do you think?
 
May 31, 2002 at 10:26 PM Post #11 of 15

sirion

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Well that was confusing; I read the typical preamp load as 27-47 Ohms, instead of 27,000-47,000..
smily_headphones1.gif


After that correction, that does make sense. For my education, though, how does that make a preamp a bad choice (What effect does that have?)?
 
Jun 1, 2002 at 9:00 PM Post #12 of 15

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Well I finally tracked one down.

Well it was better than I remember, but still not good. Using my V6's and going from the headphone jack to the out-to-amp on my Adcom GTP-400 the first thing I noticed was the obvious drop in volume. I normally keep the volume at about 9 o'clock but had to raise it to 11 o'clock to get what seems like the same volume. The only advantage it had over the jack was that it didn't have the hiss in the backgound.

Next I pluged the V6s into my Sony TC-K611S, which is the best jack on my home system. I got the volume levels between the Sony and the Adcom the same (based on hi frequency sounds) and went back and forth several times (listening to Smetana's Moldau). The bass from the out-to-amp is lacking. Again, "tinney" is what comes to mind. Almost like you were listening to it from the radio instead of a CDP (a clear radio, but radio none-the-less).

In a pinch it would work, but its not what I would want for even a backup rig.

YMMV
 
Jun 1, 2002 at 9:24 PM Post #13 of 15

Hirsch

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Ohm's Law works. If a preamp is designed to output a certain amount of current into a 47K ohm load, by dropping the load into the 32-300 ohm range, you're demanding a huge increase in current capability. If the preamp cannot supply the enormous increase in current demand, you'll get tinny sound, lack of bass, and clipping. None of this is particularly good.
 
Jun 1, 2002 at 9:37 PM Post #14 of 15

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Thanks for the post AR! I am now in the process of reconsidering my next audio purchase. I am working on a few mods for my MG Head and am wondering how it would fare as a preamp.

Thanks again

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Jun 2, 2002 at 9:40 PM Post #15 of 15

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Hey Hirsch-

Applying ohm's law to the specs for the amps,

Power amps and preamps can have about the same gain (I'm comparing the specs for Headroom's website's spec sheets to a portable mic-preamp I have). For now I'll assume this must be voltage gain and not power or current gain because everything falls together then.

Headroom headphone amps are rated at ~20 to 200 watts depending on the model and load, while preamps are rated at, well I don't know really, but it stands to reason they must be low wattage line level amps.

So by reading an amp's spec sheet, getting the gain and watts output, and the impedence of the headphones, we could get an idea of how well the system would work together...

do I have it right?
confused.gif

seems right to me!...
smily_headphones1.gif



If the gain is typically voltage gain and not power, why is this? Hmmm, bet it's because power gain depends on the load, while voltage gain does not......


sirion - if my assumptions here are right, then I led you astray in your thread when I suggested you compare the gain of your mic-pre to a typical headphone amp gain. Looks like gain is not the whole story...sorry....
rolleyes.gif
 

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