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nathan
Location: Australia
Posts: 263
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 Posted - Sat Dec 14, 2002 10:08 pm
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...An associate of mine recently purchased a joemeek preamp to match with his rode NTK...unfortunately there were impedence problems and a hum resulted- can anyone recommend a suitable preamp below US $500......already sussing out the FMRaudio RNP
much obliged -
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Graeme
Member
Location: Spain
Posts: 4663
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Posted - Sun Dec 15, 2002 12:10 am
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I'm not going to recommend anything - there are just too many units out there and you really need to to check them out for yourself. However, I would question the original premise, reference an 'impedance mismatch' causing a hum. I don't really think so.
Firstly, any mic pre-amp is, by definition, going to match a microphone's impedance - it wouldn't be much use if it didn't!
Secondly, although a mismatch (assuming one existed) might cause a loss in power transfer, it wouldn't introduce hum, just knock down the available level.
The problem is far more likely to exist in the wiring and I suggest you check that out first, rather than rushing out and spending $500 - only to find you get the same result.
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SteveG
Location: United Kingdom
Posts: 6695
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Posted - Sun Dec 15, 2002 3:49 am
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Graeme's absolutely right about the hum...
Which Joemeek preamp is it? The small one that mounts in the PC has a separate earthing strap, and if you get this in the wrong place, you could easily build a hum loop into your system, because of the way that the output is arranged to feed back to the line input on your soundcard.
The other Joemeek preamp is arranged differently, and I wouldn't have thought that this would have been such a problem.
(BTW, the only reason that I know, chapter and verse, sbout the small Joemeek is that I'm currently in the process of building one into a PC for one of my sons!)
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nathan
Location: Australia
Posts: 263
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Posted - Sun Dec 15, 2002 9:03 pm
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Thanks for the replies Graham and Steve-
sorry i dont know the exact details but Steve ( the guy who got the MQ3) spoke to a RODE technician ( we are in australia after all ) and the problem seems to be common( between the new MQ series and NTK's)...and it is to do with the auto impedence or something...but definately impedence was mentioned..maybe it has to do with the massive power supply that comes with the rode NTK..i dont know..the sound is more a very low rumble then a hum...but anyway..
So ...we tested the mic with my phonic a mixer's preamp and there is no rumble.
so can any one recommend some reputable brand preamps that we can suss out?..
once again ...much obliged
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SteveG
Location: United Kingdom
Posts: 6695
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Posted - Mon Dec 16, 2002 2:17 am
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The MQ3 is the larger external one - about which I know nothing, except that you are telling me that it doesn't work with a RODE NTK, which should be a bit worrying for Joemeek. Or perhaps it's it the RODE?
If, by some chance it is the NTK output impedance which is causing the problem, then the only sensible thing to do is to actually try it with some other preamps until you end up with a suitable one, I think. That way, at least you will have some first-hand experience of any potential problems. Do RODE themselves have an affordable recommendation to make about preamps? If they recommend one and it's no good, at least you'd have some comeback...
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Undrdog
Posts: 34
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Posted - Mon Dec 16, 2002 11:48 am
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I have the MQ-3 with a Rode's NT1-a and it works great. The MQ-3 has a CurrentSense’ mic pre amplifier - JOEMEEK’s unique pre amplifier actually optimises the performance of the microphone in circuit, meaning rich and clear sounds every time, from any microphone source.
Steve
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SteveG
Location: United Kingdom
Posts: 6695
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Posted - Mon Dec 16, 2002 1:46 pm
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| Undrdog wrote: |
I have the MQ-3 with a Rode's NT1-a and it works great. The MQ-3 has a CurrentSense’ mic pre amplifier - JOEMEEK’s unique pre amplifier actually optimises the performance of the microphone in circuit, meaning rich and clear sounds every time, from any microphone source.
Steve |
Except an NTK, it would seem... The MQ-1 also has the same preamp - it's just in a smaller box. Current sense technology is fine with relatively low-impedance mics, but can run into biassing problems with higher impedance ones. It doesn't actually 'match' the impedance - they have fixed it so that the mic output impedance becomes irrelevant by treating the output as a current rather than a voltage. In terms of reducing cable noise, etc, this is good. It's only voltage changes in cables, however caused, that tend to cause cable noise, as a rule, so if the voltage never varies, but the current does instead, then you supposedly get an advantage. Trouble is, this doesn't exactly match the noise characteristics of most preamp stage active devices too well, although this can be got around to a degree.
The trouble is that a relatively high-impedance mic output looks more like a voltage source, which really isn't going to match a current source input too well at all.
But Joemeek have got a bit of a cheek in calling this 'unique', though, because it isn't... Believe me, every single possible topology for mic preamps has been tried and used somewhere before!
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Undrdog
Posts: 34
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Posted - Tue Dec 17, 2002 10:19 am
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Maybe I should have said. have the MQ-3 with a Rode's NT1-a and it works great. The MQ-3 has a CurrentSense’ mic pre amplifier - JOEMEEK’s unique pre amplifier actually optimises the performance of the microphone in circuit, meaning rich and clear sounds every time, from any microphone source( So Say's Joe Meek). This Current Sense auto-impedance matching was the only significant change from the VC3Q. Maybe they should have left it alone.... Thanks for the Info.
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SteveG
Location: United Kingdom
Posts: 6695
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Posted - Tue Dec 17, 2002 12:47 pm
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| This Current Sense auto-impedance matching was the only significant change from the VC3Q. Maybe they should have left it alone.... |
Yes, I read that bit too, and wondered what the point was. It's slightly odd, because Ted Fletcher usually gets this sort of thing right. I had a look at the spec sheet for the op-amp he's used, and we are looking at a classic compromise situation here - the current-referred input noise is actually higher than the voltage noise below 200Hz (quite normal), and a little lower above this frequency. He seems to have traded this for the reduction in cable noise (which could be significant), and the possibility of a few problems with relatively high-impedance mic output stages.
But he didn't exactly use the quietest op-amp around for the input anyway - these preamps are built to a price, not really a spec...
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