PERSPEX QUASAR ver 2.0 prototype build

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simon
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#151 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by simon »

I'm planning on doing something similar with the GM70 amps. I reduced the cathode bypass cap to give a F3 of c100Hz to keep the bass out of the OPTs, but I intend to move this up to 400Hz which would really limit the amps for other applications. So I plan to have 2 cathode bypass caps and a toggle switch between them and earth so there's little or no voltage across the switch.
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#152 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by IslandPink »

I tried that once, but it has its limitations. You end up with a shelf in the gain, rather than a proper high-pass slope.
Remember, the output valve has a (reduced) gain associated with an un-bypassed resistor.
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Ray P
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#153 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Ray P »

This is the amplifier I'm talking about - thoughts?

Image
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#154 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by simon »

IslandPink wrote: Tue Apr 06, 2021 10:07 am Remember, the output valve has a (reduced) gain associated with an un-bypassed resistor.
Well, there would still be a bypass cap, just a much smaller one than usual. Maybe this would still affect the gain?
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#155 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by IslandPink »

Yes, at higher frequencies you get the gain of the valve with a fully by-passed resistor. At low frequencies you get the gain of the valve with just a resistor ( ie. more feedback ) which is lower. In between, you get a slope in the gain. The location of this slope depends on your choice of cap, and the resistor value, and the valve's cathode impedance.
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#156 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by simon »

Hmmm, okay, might need to think about this a little more.
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#157 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Wolfgang »

Cathode bypass cap and output cap changes will create some kind of a shelving filter curve which is not useable for xo purpose. A third option that hasn't been discussed is a simple filter in the NFB line but it's also rather a tone control than a real filter suitable for x-overs.
Just another idea (might work):
I would try to keep the existing 1uF cap and add a L and C to ground between the 6SN7 and 6C33 for a 2nd order Bessel HP filter. Bessel filters are less steep but have almost no ripple. The NFB can be easily adjusted to make up for any gain loss. The very small amount cap (nF) shouldn't affect the NFB phase and the DC R of the coil could be chosen so that it keeps the necessary minimum of the leak resistor for the 6C33. But of course it would need to be tested first and also how it sounds/changes the sound in any way. Because the amp sounds stellar as is, especially the bass.
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#158 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by chris661 »

What's wrong with putting a cap on the input?

Seems like the obvious solution, and you could build it into a short RCA cable.

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Nick
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#159 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Nick »

chris661 wrote: Tue Apr 06, 2021 6:26 pm What's wrong with putting a cap on the input?

Seems like the obvious solution, and you could build it into a short RCA cable.

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Ray P
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#160 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Ray P »

It's not as if it's going to be a tight sueeze to include a cap on the input...

Image

So an 8.8nF cap on the input will give a -3dB point ar 180Hz (assuming R1=100K) and gives a result like this:
Capture.JPG
Capture.JPG (65.38 KiB) Viewed 4743 times
Could just add another RCA socket and swap the interconnect between direct input and hi-pass input - serendipity that there are spare RCA socket holes in the chassis (just visible above the red speaker terminal)?
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#161 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by IslandPink »

Nick wrote: Tue Apr 06, 2021 6:41 pm That would be my choice.
Putting another cap in the signal path would never be my choice ( especially when it provides the HF ) :)
However I can't offer any specific suggestions for Ray's circuit re, values, because it's too small to see properly !
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Ray P
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#162 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Ray P »

IslandPink wrote: Tue Apr 06, 2021 10:37 pm ...it's too small to see properly !
Is this better;
6C33C SE-OTL Schematic.JPG
6C33C SE-OTL Schematic.JPG (61.43 KiB) Viewed 4701 times
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Ray P
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#163 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Ray P »

So the CR filter (1uF/100K) between the 6SN7 and the 6C33C gives a high pass -3dB point at about 1.6Hz - replacing the 1uF cap with a 10nF cap gives a -3dB point at just under 160Hz so perhaps a selectable cap in that position would be a better compromise to achieve the limited bandwidth without introducing another capacitor into the signal path? I guess the downside would be the introduction of a set of switch contacts but maybe not such an issue with good quality switches (especially thinking about all the contacts in the connectors, valve sockets etc.).
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#164 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Nick »

I still think adding a low pass filter inside a feedback loop is going to have a worst effect than a cap in the input.
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#165 Re: PERSPEX QUASAR ver 2.0 prototype build

Post by Nick »

BTW, what’s the point of the cathode being stood up a bit on the second triode? Is it just reducing gain a bit (for some reason).
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