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Old 9th March 2003, 09:54 PM   #21
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Quote:
The same effect can be achieved by connecting two separate channels through some resistance, it may give a pleasing effect but has nothing to do with Hi-Fi.
Not if the two channels are decoupled. Like you said facts are facts. The two output tubes are decoupled by a capacitor across the cathode resistor.

I had a similar experience experimenting with two solid state amps that shared a single battery. I did not use a cap for the battery, and got a mono signal. I used a capacitor across the battery and lo and behold stereo!

Facts are facts. My 300B amp has a 15-40kHz response, very wide stereo separation and soundstage, measurable and audible.

What is your definition of hi-fi? Now we are talking opinion and not facts.

Gabe
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Old 9th March 2003, 09:57 PM   #22
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Default Facts!

Quote:
As for crosstalk being in relation to the power supply noise, I was referring to amplitude not frequency
What i was asking is the amplitude variation across the frequency spectrum....
In your circuit as the frequency gooes down...the crosstalk will encrease...
Because of that i ask the amplitude of the crosstalk at 50 HZ.

By the way...crosstalk is not related to power suply noise!!...

Jorge
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Old 9th March 2003, 10:03 PM   #23
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Default HI-FI.

Hi,

Quote:
In your circuit as the frequency gooes down...the crosstalk will encrease...
Precisely.
Due to the decoupling caps being not perfect across the band, some coupling is going to result.
Like it or not. RC time constants will show this effect clearly.

No such thing as a free lunch,
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Old 9th March 2003, 10:06 PM   #24
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Tube_dude,

Quote:
By the way...crosstalk is not related to power suply noise!!...
Of course not, but the amplitude of cross talk is in relation to any other noise in my statement. In other words, what I meant was that one will hear more noise from the power supply before one hears crosstalk.

As for the measurement I made, it was with 20 Hz, 200 Hz, 1kHz, and 10kHz.

Crosstalk will increase for lower frequencies only if the cathode bypass capacitor is not large enough.

I will measure the 300b again this evening and give you numbers, if you wish. And I will not pad the numbers or lie. If I am wrong I will say so. However, I do not think it will be quite the same with the 2A3.

Oh, I did say I wouldn't add more input didn't I.

Gabe
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Old 9th March 2003, 10:08 PM   #25
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Quote:
Due to the decoupling caps being not perfect across the band, some coupling is going to result.
Fdegrove,

Of course this is true. I do not argue that. But I do argue that your comment about severity is off base, since I measure much lower than expected.

We'll see later.

Gabe
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Old 10th March 2003, 01:15 AM   #26
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You in effect have a positive bias on one tube and a negative on the other. That is why i said non polarized. It was just a thought. If it works and sounds good the arithmetic is a moot point
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Old 10th March 2003, 02:29 AM   #27
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Default results of testing

OK all,

Here are the results of my test: the amp is a 300b SE amp using a common DC power supply with both filaments in parallel, with a 500 ohm pot connecting the ends, and a 360 ohm common cathode resistor bypassed by a 47µF cap (note, not 470µF as I recommend in the 2A3). Both channels have a non-inductive 8 ohm load. The right channel is driven to full output just before clipping. This is measured at the output.

right channel

20 Hz 15Vp-p
200Hz "
2kHz "
20kHz "

left channel

20 Hz 5Vp-p
200Hz 1Vp-p
2kHz 0.5Vp-p
20kHz 0.5vp-p

So as expected, lower frequencies had more crossover... but WAIT! I put another 47µF cap across the first one. The results?

Right Channel

20 Hz 15Vp-p
200Hz "
2kHz "
20kHz "

Left Channel

20 Hz 2Vp-p
200Hz 0.5Vp-p
2kHz 0.5Vp-p
20kHz 0.5Vp-p

Note that the crosstalk was halved at the lower frequencies. This proves that the audio is being shunted by the cathode capacitor at those frequencies.

Notice the fact that the higher frequencies haven't changed. I suspect that those crosstalk responses are more from the power supply, or even proximity of the audio transformers to each other and to the tubes, than anything else.

Of course, since bass frequencies are omnidirectional as everyone knows, it doesn't really matter.

A 15 volt to 0.5 volt difference is pretty large. I calculated about 39dB (using the 20LogA. 15/0.5 being the voltages used) . Most phono cartridges can not do better than 24dB, the best I have seen at about 29dB, that at 1kHz, dropping to low levels at both ends of the spectrum.

I took pictures of my scope of both responses, with 47 and 94µF capacitance. I will put it on my web site soon.

Gabe
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Old 10th March 2003, 12:10 PM   #28
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Valveluver,

I understand what you mean, but that would be true if the common point of reference were in relation to the filament supply, but it isn't. It is in relation to B+ ground and the grids. So it is positive with respect to both grids.

Thanks for the input, though. Good point!

Gabe
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Old 10th March 2003, 12:29 PM   #29
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Default Re: The Stereo SE...

Quote:
Originally posted by Tube_Dude
The only sugestion that i can make...is that the shared catode resistor common to the 2 chanels will introduce same crosstalk and consequent intermodulation in the low frequences...
It's negligable, and likely to be the same as any crosstalk introduced by both channels sharing the same chassis. This simply isn't an issue with a 470MF bypass cap.

Tubedude and Frank - I wonder what you would say about my DHT SE amps where the cathodes of ALL the tubes are directly grounded! Including the preamp tubes. Surely they are "only seperated by the filament resistance"?
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Old 10th March 2003, 10:29 PM   #30
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Default SHARED CATHODES.

Hi,

Quote:
Tubedude and Frank - I wonder what you would say about my DHT SE amps where the cathodes of ALL the tubes are directly grounded! Including the preamp tubes. Surely they are "only seperated by the filament resistance"?
That anyone would do that on a monoblock I can still understand,
what I also understand is that Gabe would defend this on a commercial basis.

This is something I would never do for my own use though, guess we just have different understandings of what the better audio is about.

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