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Old 1st Jun 2020, 8:26 pm   #1
Welsh Anorak
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Default Quad II sensitivity

Hi
I've been using my Quad IIs with a transistor pre-amp with no problems. However I'm trying another pre-amp and they are far too quiet. The output spec is 1v/1.2k. Is it feasible to increase the sensitivity of the Quads? The only obvious way I can see is changing the values of the resistors in the feedback loop (R10 and R11). However I'm not sure this would be a good idea. What do you think?
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Old 1st Jun 2020, 8:54 pm   #2
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Default Re: Quad II sensitivity

Is it possible to increase the output from the preamp?

Personally, I wouldn't fancy altering the Quad's circuit.

Cheers

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Old 2nd Jun 2020, 9:19 am   #3
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Default Re: Quad II sensitivity

Maybe you could build a simple, single EF86 flat gain stage. There is ample power in the QUAD II for heater and HT
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Old 2nd Jun 2020, 9:57 am   #4
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Default Re: Quad II sensitivity

Thanks - I would prefer to do something like this rather than tamper with the feedback. Otherwise I could build a transistor based one as it's being fed from a solid state preamplifier after all!
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Old 2nd Jun 2020, 10:53 am   #5
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Default Re: Quad II sensitivity

I see no reason to change anything on the Quads as they work perfectly well with the other pre-amp. What needs alteration has to be raising the lower output of the second pre-amp. What make/model is it?
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Old 2nd Jun 2020, 8:35 pm   #6
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Default Re: Quad II sensitivity

It's a Yamaha 7.2 surround amplifier (RX-V667) with a pre-amp output. There is a + and- 10dB variation in the levels allowed, but that still isn't enough to balance the outputs. However I'm not going to interfere with the Quads (or the Yamaha for that matter) and will simply route the signals through the existing preamp.
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Old 3rd Jun 2020, 7:02 am   #7
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Default Re: Quad II sensitivity

Simple low fixed gain stage would be one solution, could be retro fitted to the Yamaha on a separate board either that or do it with valves near the Quad 2 powered off the Q2, cathode follower buffer IP might do on it's own or add a low gain stage,. Being outside the FB loop it shouldn't make much difference if implemented well. Nice little project.

Andy.
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Old 3rd Jun 2020, 8:47 am   #8
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Default Re: Quad II sensitivity

Quote:
Originally Posted by rontech View Post
Maybe you could build a simple, single EF86 flat gain stage ...
This would work. But the EF86 is optimised for high gain. The OP needs precious little gain (<10dB ?) so triodes would probably be quieter and perhaps more linear (of course you could always triode-connect the EF86).

Cheers,

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Old 3rd Jun 2020, 9:02 am   #9
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Default Re: Quad II sensitivity

Quote:
Originally Posted by GrimJosef View Post
Quote:
Originally Posted by rontech View Post
Maybe you could build a simple, single EF86 flat gain stage ...
This would work. But the EF86 is optimised for high gain. The OP needs precious little gain (<10dB ?) so triodes would probably be quieter and perhaps more linear (of course you could always triode-connect the EF86).

Cheers,

GJ
Use one half of an ECC82 maybe?

P.S. For playing my 78's with an Ortofon cartridge I use an old NAD head amp into an old Quad potentiometer then into a Quad 303 power amp it should work with the QuadII I would think.
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Old 3rd Jun 2020, 9:30 am   #10
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Default Re: Quad II sensitivity

If you're playing 78's then you won't need to worry about noise from the electronics .

Cheers,

GJ
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Old 3rd Jun 2020, 11:00 am   #11
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Default Re: Quad II sensitivity

Hi Glyn, I,ve used this op amp cct several times with good results. It is intended to give a balanced OP from an unbalanced source but yiu can omit the second op amp and use the output from the 1st opamp. the 10K pot gives a smooth gain adjustment, I used blue 10 turn pots from the bay.The opamps need 12/15v + and - but I have used a divider from a single supply to derive half supply to feed the non inv inputs. I initially tried it with a single 5v supply but the output clipped very readily A bit of thought showed that line level outputs can have very high peaks!


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Old 3rd Jun 2020, 11:24 am   #12
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Default Re: Quad II sensitivity

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Originally Posted by GrimJosef View Post
If you're playing 78's then you won't need to worry about noise from the electronics .

Cheers,

GJ
Thanks GrimJosef.

Normal valve noise I agree
.
I do have a very old QUAD 33 preamp which I used for a while for the useful filtering facilities. However there was slight audible hum and a tendency for noisy pots, edge connectors and switching.

Servisol worked for a time but the unit needs a thorough overhaul .

Many of my 78's have surprisingly quiet surfaces and I am not inclined to spend time and money on the Quad.
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Old 3rd Jun 2020, 2:26 pm   #13
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Default Re: Quad II sensitivity

As usual you've given me plenty to think about! The op-amp approach is tempting, but I rather like the idea of an extra valve in its own little box behind each Quad, especially as powering then wouldn't be an issue via the Jones plugs.
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Old 3rd Jun 2020, 3:30 pm   #14
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Default Re: Quad II sensitivity

Anything optimised for high gain will be fine, and better than a dedicated low gain no feedback one, at low gain with negative feedback, e.g. an unbypassed cathode resistor job.
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Old 3rd Jun 2020, 6:05 pm   #15
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Default Re: Quad II sensitivity

It's not often that I would say this about a valved device - but however well you build, screen and connect a powered EF86 type pre-amp, I think you'll have some difficulty in achieving the low noise floor of a half-decent transistorised pre-amp. Have you tried the low-priced (c.45.00) NAD or Project preamps?
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Old 3rd Jun 2020, 9:18 pm   #16
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Default Re: Quad II sensitivity

I have Merlin Blencowe's book on Hi Fidelity Pre amps. There is a lot of information on various EF and ECC valves and their use circuits with measurements of noise and distortion data. Most instructive.
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Old 3rd Jun 2020, 11:27 pm   #17
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Default Re: Quad II sensitivity

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Originally Posted by Edward Huggins View Post
... however well you build, screen and connect a powered EF86 type pre-amp, I think you'll have some difficulty in achieving the low noise floor of a half-decent transistorised pre-amp ...
True. But the OP has a fraction of a volt of signal to feed into this valve. I don't think he's going to need super-low noise performance.

Cheers,

GJ
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