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Old 8th May 2003, 12:48 PM   #1
Elkaid is offline Elkaid  Canada
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Quote:
Oh, and don't connect your passive preamp to a gainclone.
uh ? Is there any reasons ?
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Old 8th May 2003, 01:01 PM   #2
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Wink The reason

Quote:
Originally posted by Elkaid


uh ? Is there any reasons ?

Well, basically because I don't like passive preamps.
Or you use very good (expen$ive) and short interconnects, or the sound degrades.
Anyway, the Gaincards have volume pots (actualy rotary switches with resistors), but from there goes a very short piece of wire to the chip.
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Old 8th May 2003, 01:10 PM   #3
Elkaid is offline Elkaid  Canada
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oh !

I though you were making reference to something more "subtile" such as input impedance etc.

Thank you !
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Old 8th May 2003, 05:20 PM   #4
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Default Re: The reason

Koinichiwa,

Quote:
Originally posted by carlosfm



Well, basically because I don't like passive preamps.
Or you use very good (expen$ive) and short interconnects, or the sound degrades.

A competently engineered passive Linestage has no problems with either output impedance/load OR with undue loading of the source. It is really easy.

Sayonara
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Old 8th May 2003, 05:34 PM   #5
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Default Re: Re: The reason

Quote:
Originally posted by Kuei Yang Wang
A competently engineered passive Linestage has no problems with either output impedance/load OR with undue loading of the source. It is really easy.
As it happens, I'm working on a power amp which has a 200 ohm input impedance. I'd like to see a passive which could drive it without severe loading losses (no more than -3dB).

se
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Old 8th May 2003, 05:41 PM   #6
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Koinichiwa Steve,

Quote:
Originally posted by Steve Eddy


As it happens, I'm working on a power amp which has a 200 ohm input impedance. I'd like to see a passive which could drive it without severe loading losses (no more than -3dB).

se
Most active preamplifiers will not tollerate such an extreme load, methinks you'll have to buffer the input, maybe with a 4:1 stepdown transformer, in which case the input Z is a more managable 3200 Ohm.... Or you could just use a BUF634... ;-)

Sayonara
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Old 8th May 2003, 07:04 PM   #7
mikelm is offline mikelm  England
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Default low i/p Z

Quote:
Originally posted by Steve Eddy


As it happens, I'm working on a power amp which has a 200 ohm input impedance. I'd like to see a passive which could drive it without severe loading losses (no more than -3dB).

se
Would that be a gainclone inverting i/p power amp with 200 ohms i/p Z or a different variety ?

I reduced my + input resistance from 10k to zero ohms and liked the results very much. Now I am interested to discover how lowering the feedback Z will affect the sound.

has anyone tried this ?

mike
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Old 8th May 2003, 07:26 PM   #8
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Default Re: Re: Re: Re: The reason

Quote:
Originally posted by Kuei Yang Wang
Most active preamplifiers will not tollerate such an extreme load, methinks you'll have to buffer the input, maybe with a 4:1 stepdown transformer, in which case the input Z is a more managable 3200 Ohm....
But if you use a 4:1, then most all of the voltage gain of the amp (which is using a 1:5) will be lost. At that point, you just might as well just use a high current buffer and slap it on the preamp and drive the speakers from there.

Quote:
Or you could just use a BUF634... ;-)
Actually I've already got a preamp for it. Just that your original post made it sound as if you had some way to drive such a load passively without loading loss.

se
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Old 8th May 2003, 07:40 PM   #9
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Koinichiwa Steve,

Quote:
Originally posted by Steve Eddy


But if you use a 4:1, then most all of the voltage gain of the amp (which is using a 1:5) will be lost.

I figured you where using a stepup input transformer, but why on earth one with 200 ohm input impedance? How about a 5k:80k (1:4) which gives gain and much saner impedances. A 200 Ohm input impedance will not even sit well in Pro Audio systems.


Quote:
Originally posted by Steve Eddy


Actually I've already got a preamp for it. Just that your original post made it sound as if you had some way to drive such a load passively without loading loss.

Sure do. Just use a 600 Ohm version of a TVC driven from a source that can tolerate 200 Ohm worst case load.... ;-)

BTW, I have driven a nominally 600 Ohm input of a transformer (S&B TX-905 1:4 10k secondary) from a nominally 10k TVC with no ill effects, but the secondary was unloaded... It simply had too much gain for my taste and I switched to a 877 as 10k:10k or 10K:40K selectable.

Sayonara
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Old 8th May 2003, 08:19 PM   #10
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Default Re: Re: Re: Re: Re: Re: The reason

Quote:
Originally posted by Kuei Yang Wang
I figured you where using a stepup input transformer, but why on earth one with 200 ohm input impedance? How about a 5k:80k (1:4) which gives gain and much saner impedances. A 200 Ohm input impedance will not even sit well in Pro Audio systems.
Well, seeing as this particular version of the amp isn't intended to be used with other equipment, the 200 ohm input impedance wasn't an issue.

The reason it is 200 ohms is due to something of a compromise in keeping the number of components down to just seven.

The transformer is "normally" intended to have a 39.2k load on it which gives a 1.5k ohm input impedance. But in this case, it's driving a bipolar emitter follower so base current is something of an issue.

I actually prefer a little bit of current through the secondary of the transformer but when I opted to go with a resistive load rather than a constant current source for the follower, the bias current required tripled and of course so does the base current.

Going with the 4k load relieves the transformer secondary of some of the bias current. But of course leaves you with a 200 ohm input impedance.

Of course I could just keep the 39.2k load and use a resistor from the base to the positive rail and determine how much base current goes to the secondary, but then that would be 8 parts, not 7.

Quote:
Sure do. Just use a 600 Ohm version of a TVC driven from a source that can tolerate 200 Ohm worst case load.... ;-)
Yeah? Since when did you start recommending TVCs?

Quote:
BTW, I have driven a nominally 600 Ohm input of a transformer (S&B TX-905 1:4 10k secondary) from a nominally 10k TVC with no ill effects, but the secondary was unloaded... It simply had too much gain for my taste and I switched to a 877 as 10k:10k or 10K:40K selectable.
Yeah. Just depends on what you're looking to get in the end.

se
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