|
Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
|
Thread Tools |
1st Jun 2020, 8:26 pm | #1 |
Dekatron
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,927
|
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? |
1st Jun 2020, 8:54 pm | #2 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,039
|
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 Aub
__________________
Life's a long song, but the tune ends too soon for us all. |
2nd Jun 2020, 9:19 am | #3 |
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
|
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
__________________
Time flies like an arrow, fruit flies like a banana |
2nd Jun 2020, 9:57 am | #4 |
Dekatron
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,927
|
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!
|
2nd Jun 2020, 10:53 am | #5 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,326
|
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?
|
2nd Jun 2020, 8:35 pm | #6 |
Dekatron
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,927
|
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.
|
3rd Jun 2020, 7:02 am | #7 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,667
|
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.
__________________
Curiosity hasn't killed this cat...so far. |
3rd Jun 2020, 8:47 am | #8 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Quad II sensitivity
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
__________________
http://www.ampregen.com |
3rd Jun 2020, 9:02 am | #9 |
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
|
Re: Quad II sensitivity
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 Quad II I would think.
__________________
Time flies like an arrow, fruit flies like a banana |
3rd Jun 2020, 9:30 am | #10 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Quad II sensitivity
If you're playing 78's then you won't need to worry about noise from the electronics .
Cheers, GJ
__________________
http://www.ampregen.com |
3rd Jun 2020, 11:00 am | #11 |
Tetrode
Join Date: Jul 2016
Location: Renfrew, Renfrewshire, UK.
Posts: 93
|
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!
https://www.vintage-radio.net/forum/...1&d=1591178363 |
3rd Jun 2020, 11:24 am | #12 | |
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
|
Re: Quad II sensitivity
Quote:
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.
__________________
Time flies like an arrow, fruit flies like a banana |
|
3rd Jun 2020, 2:26 pm | #13 |
Dekatron
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,927
|
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.
|
3rd Jun 2020, 3:30 pm | #14 |
Guest
Posts: n/a
|
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.
|
3rd Jun 2020, 6:05 pm | #15 |
Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,338
|
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?
__________________
Edward. |
3rd Jun 2020, 9:18 pm | #16 |
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
|
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.
__________________
Time flies like an arrow, fruit flies like a banana |
3rd Jun 2020, 11:27 pm | #17 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
|
Re: Quad II sensitivity
Quote:
Cheers, GJ
__________________
http://www.ampregen.com |
|
9th Nov 2020, 4:06 pm | #18 |
Tetrode
Join Date: Nov 2020
Location: Edinburgh, Scotland, UK.
Posts: 89
|
Re: Quad II sensitivity
You could pick up an inexpensive audio mixer such as those made by Behringer or Soundcraft and put this between the two units to step up the gain as required .
|
10th Nov 2020, 12:13 am | #19 |
Heptode
Join Date: Oct 2019
Location: Täby, Sweden
Posts: 708
|
Re: Quad II sensitivity
The OP said the preamp output was 1V - that does not sound particularly low to me. Could it be another issue, for instance the impedance matching to the speakers? Or the speakers themselves?
|
10th Nov 2020, 12:21 am | #20 |
Octode
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
|
Re: Quad II sensitivity
QuadII has a normal sensitivity of 1.4V for full output.
Its one of the reasons why its such a quiet background noise amp, so it requires a fairly high gain preamplifier. And the quad 22 control unit gave that gain so everything was well matched. A. |