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Old 13th May 2013, 01:47 AM   #1
borg7x9 is offline borg7x9  New Zealand
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Default Impedance matching pre -> power.

Hi All, this is my first post, so please be gentle :-)

I recently bought a pair of PSVANE 845-T monoblock power amps (on a bit of a whim ) which have a transformer loaded input presenting a load of 600ohms ( Se and balanced ). The manufacturers "matching" pre amp is a transformer design also with an out put impedance of 600 ohms. I have not bough this (yet).

I am largely failing to drive them very well with the pre-amps I have on hand. One worked very well( see list below) but I've not tried a transformer coupled pre-amp as the designers intended.

I'm not sure I want to buy their pre amp. Though I might, if I thought it would work well. I am not convinced that it will given my findings so far. Which is ...... The lower the output impedance of the pre-amp the better they sound.

So far I've loved the Einstein "the tube" with them. Its an incredible preamp, but expensive. the only way I can try the PSVANE matching pre-amp is to buy it, but I need convincing that it should work well. Sooooo........

..........would it be possible for someone to explain to me how a 600ohm output pre - > 600ohm input power can work well, when most of the info and my experience, tells me a rule of thumb of 10x input to a given input resistance works the best ?? Does this rule of thumb not apply to preamps of transformer output designs ??

FYI Things I've tried

1. Plinius m8 solid state = terrible , no bass at all, surprised me supposed 100ohm out
2. Schitt lyr pre out = not bad bass ( 75ohms out ) but too noisy overall:-low quality.
3. Cat SL1 ultimate = good but a little bass light bottom octaves (100ohms out)
4. Einstein " the tube" = magnificent. (50ohms out).

Thanks for your time and advice.

Myron.
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Old 13th May 2013, 05:12 AM   #2
godfrey is offline godfrey  South Africa
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Expensive whim!

600 Ohm input impedance is somewhere between bizarre and perverse. I'm not surprised some preamps struggle to drive it. Presumably their preamp works well with it, since they recommend the two be used together.

Unfortunately that's got it's own quirks too - you have to reach round the back to get to the input selector. I wouldn't want to live with something like that, but maybe they figure anybody who wants a valve amp must be a masochist anyway, and will enjoy the extra inconvenience.

Re impedance matching:
It's normal in domestic audio equipment to have low output impedance and high input impedance, but it doesn't have to be that way. However since high input impedance is the norm, some preamps (whatever their output impedance) don't cope well with a low impedance load, as their designers may not have seen that as a requirement.

Preamps with output coupling capacitors are likely to suffer from "no bass" syndrome, while those with direct coupled or transformer coupled outputs should at least clear that hurdle.
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Old 13th May 2013, 07:23 AM   #3
borg7x9 is offline borg7x9  New Zealand
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Hi Godfrey, Thanks for the reply.
I got them at a particularly attractive price and to be fair they do sound very, very good with the Einstein preamp.(driving big tannoys) Unfortunately the Einstein is about 20k usd new so out of my price range. I could keep an eye out for a second hand one but there are not many of them about. I don't mind about the lack of remote or the switch on the back. I listen to mainly records with some digital so two inputs is enough. What I am worried about is their preamp wont work very well given that the 600ohm output impedance matches the input impedance of the power amp. It is not usual (normally 10x input as output) in my experience, but then, it seems that transformer coupled pre and power amps are not the norm either. I want to know if it is possible that they could work well with these specs.

They have offered to refund my purchase if i don't get on with it, but I'd be in the hole for about $400 USD shipping if I need to send back.

Any thoughts are appreciated.

Thanks.
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Old 13th May 2013, 09:10 AM   #4
DF96 is offline DF96  England
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600 ohms is a standard for driving long lines in professional audio. No point whatsoever in using it for short connections. Much better to use the normal domestic standard of low output impedance and high input impedance.

Also, remember that output impedance and optimum load impedance are quite different concepts - but often confused, even by manufacturers. A typical preamp might have an output impedance of a few hundred ohms, yet an optimum load impedance of 10-20k. Such a preamp cannot drive 600 ohms without serious distortion and (probably) bass loss.
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Old 13th May 2013, 10:10 AM   #5
borg7x9 is offline borg7x9  New Zealand
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DF96, those are my thoughts, it doesn't make sense to me that an impedance matched design (600 ohms output and input) would make a good match given that my experiments so far have shown that only the 50ohm Einstein has been what I would call "brilliant" in the bass region. I was wondering if the transformer coupled design might mean that the 600-600 matched relationship made sense in that model.
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Old 13th May 2013, 10:11 AM   #6
FoMoCo is offline FoMoCo  United States
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600 ohms came from the telephone system. Telephone lines have an impedance of somewhat near 600 ohms and were used to connect audio from the studio to a transmitter. It's just stuck around as a professional standard.
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Old 13th May 2013, 11:33 AM   #7
AndrewT is offline AndrewT  Scotland
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The input might be specified as 600ohms compatible.
This, if true, would mean the pre-amp driving this input should have around a 600ohm source impedance.

If you read Jung, you will find some very good primers on balanced impedance connections. In there you will find Jung explaining why a VERY LARGE ratio of Zin to Rs is important to a balanced impedance connection.
10ohms as a source impedance combined with 10Mohms of input impedance (a transformer) automatically gives good interference rejection. Matching of the impedances then improves the interference rejection.
Worth reading Self and Rane.
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Old 13th May 2013, 11:56 AM   #8
Pano is offline Pano  United States
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Quote:
Originally Posted by godfrey View Post
600 Ohm input impedance is somewhere between bizarre and perverse.
Ditto! What a weird idea.

Quote:
Originally Posted by DF96 View Post
600 ohms is a standard for driving long lines in professional audio.
Yeah, very long lines. Like miles. Most pro gear has a much higher input impedance even on the mic inputs. Line input impedance is typically 10K. Low impedance is great for noise rejection, most noise will have a very hard time driving 600 ohms as well. But as others have already mentioned, it does not make sense for domestic use.

If it were my amp, that input section would be the first thing I'd rip out. But you may not feel safe working on tube gear and high voltages.

Of course a 600 ohm balanced input makes perfect sense when one of the few preamps that will drive it is the matching preamp you also sell.
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Old 13th May 2013, 12:23 PM   #9
borg7x9 is offline borg7x9  New Zealand
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FoMoCo. Thanks for the info...As a telco engineer I am familiar with that. A lot of the network was built around 600ohms matching to maximise carry and to avoid reflection. I've not come across it in Audio before. Other than pro kit with microphones etc.

Andrew, Thanks for the tips. You are right, I really should read up on the Maths. I suppose this forum is a bit of a short cut to that. Now to find a copy in my local library..Hmmmmm. maybe the internet :-)
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Old 13th May 2013, 12:29 PM   #10
borg7x9 is offline borg7x9  New Zealand
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Quote:
Originally Posted by Pano View Post

Of course a 600 ohm balanced input makes perfect sense when one of the few preamps that will drive it is the matching preamp you also sell.

:-). Quite right..... I had considered that strategy.

I could always get my local tube prof to change the input transformer to something more "Normal" Maybe that's the best way to go.
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