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Volume Control

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Posted on June 17, 2015 at 18:57:28
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 Hello everyone,

I am trying to set up a dedicated mono system but I have reached a point where I just don't have the knowledge to proceed. I have a Rek-O-Kut L34 with an Audio Technica ATP-12T and a GE RPX cartridge. I have a, don't laugh, Lafayette preamp which uses two 12AX7's, a Quad II power amp and a DIY Hedlund horn full range speaker. My problem is this, the Lafayette does not have a volume control and nor does the Quad II. How can I go about installing a volume control somewhere and maintain cartridge loading? Thanks in advance.

Jim

 

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RE: Volume Control, posted on June 17, 2015 at 19:21:53
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Looks like you don't have a preamp, you have a phono preamp. Either that or you haven't located the knobs yet, which seems unlikely :^)

If I am right, then you still need a control center of some sort. In the old days, that would be source switching, volume control, and tone controls. Many modern units leave out the various tone controls.

 

RE: Volume Control, posted on June 17, 2015 at 23:01:52
gilmorneau
Audiophile

Posts: 288
Location: Colorado
Joined: August 10, 2004
 I think Paul's right. You have a phono preamp (aka a phono stage), which is only one component of a full function preamp. You can proceed a couple of different ways. You could add a line stage + source selector, which would then give you all the features of a preamp. Or you could just get a mono full function preamp. The Quad QC2 would be the obvious choice if you already have a mono Quad II amp. It would plug right in, and give you everything you need to start listening.

If you decide to just add a line stage between the Lafayette phono pre and the Quad amp,a DIY unit might be a good option, since most vintage (especially mono) preamps had phono stages built right in, which would just be redundant since you already have the Lafayette. For a DIY line stage, it's hard to beat the one in the back of the RCA tube manual for simplicity and good sound. Sure, there's 'better', but for the system you've described, it would be terrific. Note that the value of the volume pot at the input would depend on what the Lafayette's output likes to see.

BTW, the cartridge loading is determined at the input of the Lafayette, but the volume control would likely go at the input of the line stage (i.e. at the output of the Lafayette), and so wouldn't affect the cartridge.

 

RE: Volume Control, posted on June 18, 2015 at 09:42:30
BofService
Audiophile

Posts: 741
Location: Atlanta
Joined: February 28, 2003
 Not sure if the gain and impedance values between the Lafayette and the Quad would support it, but you can very inexpensively build yourself a passive volume control between them with just 4 RCA connectors and a stereo volume pot wired together. Search on the internet to find out how to wire a volume control. A small metal or plastic candy tin would be adequate to mount the parts in. You could do the whole thing for about $5 (assuming you are already set up to solder). I have a couple of these at home I use for testing and that usually works fine. Get a stereo 50k or 100K audio taper potentiometer and give it a try.

Or get a real preamp as several have suggested.

Gary

 

RE: Volume Control, posted on June 18, 2015 at 10:38:25
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Very often, the O/P impedance of tube equipment is too high for a passive control center to be satisfactory. :>(

Let's say that a 50 Kohm load is needed by unit X. That's easy enough to do with a 50 Kohm control. The problem is downstream from the pot., where a 500 Kohm I/P impedance is in order. To avoid high freq. roll off, with that sort of I/P impedance, a pentode or cascode I/P stage (low Miller capacitance) is indicated. Lots of power amps have 100 Kohm impedance triode I/P stages and, therefore, mate badly with a 50 Kohm passive control setup. Keep the 1:10 impedance rule firmly in mind.

The 1st order of business is working out the amount, if any, of line stage gain needed. Then, sensible advice about line stage configuration can be provided.

BTW, the "classic" RCA passive EQ phono circuit requires a minimum of a 220 Kohm impedance downstream. The 6SL7s, 6SC7s, and 12AX7s found in early mono mag. phono preamps are all wimps that can't drive loads.


Eli D.

 

RE: Volume Control, posted on June 18, 2015 at 11:46:28
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 The Lafayette provides 40db gain so that should be fine.

Thank you,
Jim

 

RE: Volume Control, posted on June 18, 2015 at 11:49:00
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 I have wired a 100K audio taper pot between the phono stage and the power amp. It works fine but the output sounds very harsh to me, hence the impedance question.

Thank you,
Jim

 

RE: Volume Control, posted on June 18, 2015 at 11:56:02
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 I also found the following information in the Quad II power amp specs,
Input Sensitivity: 1.4V rms for 15W output

Load impedance: 1.5M ohm in parallel with 10pF

The input is d.c. coupled to V1 which can cause problems with pre-amps other than QC22 due to a d.c. voltage on their output

I have no idea how this effects things?!

Thank you,
Jim

 

RE: Volume Control, posted on June 19, 2015 at 07:25:03
gilmorneau
Audiophile

Posts: 288
Location: Colorado
Joined: August 10, 2004
 just to clarify, I was suggesting the *line* stage from the back of the RCA tube manual, which uses a single 12AU7, not the phono stage, which uses the 12AX7 and requires a load of 220k or more. The OP's mono system would then be: Lafayette phono preamp---->RCA line stage---->Quad II amp. I've used this line stage with Quad II's (though I used octal tubes), and it works very well. The Quad II has an input impedance of 1.5meg ohms IIRC, which is easily driven by a 12AU7 (or 6SN7) cathode follower. The input impedance of the line stage is set by the volume pot, which is spec'd at 100k in the RCA tube manual. I don't have a schematic of the Lafayette phono pre, so I couldn't say whether it would drive 100k. If it takes the output off the plate of the 12AX7, probably not, but if it has a CF output, then it shouldn't pose a problem.

The easiest and most elegant solution would still be to source an old mono Quad preamp. They show up on auction sites once in a while (hint: try in the U.K.), and don't seem to go for a lot of money.

 

Lafayette schematic, posted on June 19, 2015 at 12:55:06
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005



Hello all,

Here is a schematic. I hope it helps. I have a NAD 1240 preamp with a few line in/line out's so I may try that also.

Thank you,
Jim

 

RE: Lafayette schematic, posted on June 19, 2015 at 20:11:38
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 That setup uses AC heating. You need tubes with a hum bucking heater to have any chance at an acceptable residual hum level. The obvious choice is buying culled phono grade Sovtek 12AX7LPSes from a reliable dealer, like AA sponsor Jim McShane.

The 0.01 μF. O/P coupling caps. combine with the downstream I/P impedance to form a high pass filter. FYI, those caps. combine with 100 Kohms to "corner" at (sic) 159 Hz. Working into 500 Kohms gets the "corner" down to a marginal 31.8 Hz. Don't connect the Lafayette unit to a SS line stage, unless modifications are made.

I hope I'm wrong, but it seems that unit was intended to be used with old GE carts., whose internal inductance contributes to the RIAA EQ. Another "gotcha" is the I/P impedance. Modern MM level carts need a 47 Kohm I/P impedance, not 6 Kohms.


Eli D.

 

And the 2nd Stage is Grid Leak Bias :(, posted on June 19, 2015 at 20:57:52
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 That preamp needs a lot of help.


 

RE: And the 2nd Stage is Grid Leak Bias :(, posted on June 20, 2015 at 09:37:35
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



Both stages use grid leak, AKA contact, bias, which is OK at these low signal levels. Thorsten Lösch pointed out that grid leak bias in the 2nd stage lightly loads the EQ network, for better bass extension, in a passive EQ phono preamp setup. I exploited the idea in the tweaked RCA setup. However, I have my doubts about the 3M grid leak resistors Lafayette used, in providing a sufficient contact potential.

The Lafayette unit is a stereo rehash of GE's mono 6SC7 setup, which employs active EQ. Remember, GE was 1st with their moving iron cartridges.

When you get down to brass tacks, all tubed phono preamps are either RCA style (passive EQ) or GE style (active EQ).


Eli D.

 

Lafayette Phono Preamp, posted on June 20, 2015 at 10:11:53
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Aside from being stereo, there is much to be done, in whipping that old unit into shape, for use with modern MM level cartridges.

A lot of what you need to know can be found in the thread linked below and, in turn, the imbedded link(s).


Eli D.

 

Chrome Cutie Preamps have .1 uf final cap; not .01...Hum is easily controlled..., posted on June 21, 2015 at 08:56:11
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
Joined: October 4, 2006
 The high fidelity performance potential is great with these units...More info soon...Regards...

 

RE: Chrome Cutie Preamps have .1 uf final cap; not .01...Hum is easily controlled..., posted on June 21, 2015 at 10:47:11
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 That the schematic provided is favorably wrong is good news. The factor of 10 is important.

IMO, aside from using 7025 equivalents, to control hum, biasing the filament winding's CT off B+, instead of a direct connection to ground, is indicated.

Speaking of B+, replacing the wretched 1/2 wave rectifier with a low noise full wave bridge seems obvious to me. Other than the routine replacement of dried out electrolytic caps., the CRC PSU filter seems quite adequate.

FWIW, I still would not connect the Lafayette preamp to a SS line stage, without adding buffering between the 2 units.


Eli D.

 

The Chrome, Cute Preamps Are Challenging to Sound Right..., posted on June 21, 2015 at 11:02:38
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
Joined: October 4, 2006
 While I treasure these preamp icons, they do require some hard work to get right. They have little room inside; especially with the tight innards of the stereo units. Their potential is VERY REWARDING SONICALLY; even if simply upgrading the power supply, tweaking the Active Phono EQ and correcting the input resistance.

Having upgraded more than a few of these, I must warn you that every unit's original final coupling caps leaked DC voltage at the output. Connected to an amp with no input cap in the amp can quickly sound harsh, as leaking DC changes the bias of the next stage. The resulting sound can get fuzzy quickly. Rather than decipher one system installment, as other posters have already, perhaps it is the right time for me to start a new thread about these cute, chrome preamps; which I will post above...Best Regards & ENJOY !

 

RE: Chrome Cutie Preamps have .1 uf final cap; not .01...Hum is easily controlled..., posted on June 21, 2015 at 14:32:31
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 Thank you Eli. I will weigh my options. I am in fact using a vintage GE RPX cartridge.

Jim

 

RE: The Chrome, Cute Preamps Are Challenging to Sound Right..., posted on June 21, 2015 at 14:33:41
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 Thanks a bunch!

Jim

 

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