LCR Phono using 6C45P

the road not taken.
Naz
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LCR Phono using 6C45P

Post by Naz »

I modified my current (RC RIAA) Phono / Pre to use a 600 ohm LCR and although I haven't actually built this version I expect it to perform very well since its based on a design that measures (and performs) exactly as predicted.

Note 1 - The reverse RIAA in the front end is used only to plot the real world frequency response in the Sim. My thanks to Jim Hagerman, it was borrowed from his site.

Note 2 - The model for the 600 ohm LCR was borrowed from Tango but other equivalents such as S&B / Silk should work with similar results. You may want to check that your LCR will handle DC current of a few mA. Alternatively, use a different model utilising DC blocking or add it yourself. Suggestions here Dave?

Note 3 – The RC network across the LCR is used to compensate for falling top end response, a trait of the LCR networks it seems. My choice, but those who do not agree may choose to omit this.

Note 4 – The Grid Choke is tuned with the coupling cap to restore bottom end frequency response arising from not terminating the LCR with 600 ohms. This technique significantly improves gain over correct termination and does not appear to cause other problems. LF response can be tailored with different tuning.

Note 5 – The Phono stage has sufficient gain to directly couple a low OP MC cartridge, though some may opt to use a SUT (I’m not one of them because IME all but the very best are BW limited to some degree).

The Line stage is optional and I’ve chosen Lithium battery bias in the Grid circuits for something different. It works very well. I’ve chosen Schottky diodes to bias both Phono stages. I haven’t actually tried this on the 2nd stage but anything else seems compromised on the 6C45P, including a single (standard) diode equivalent … a result of higher dynamic impedance I suspect.

Obviously tube selection and many other parts of the circuit could easily be changed to suit individual taste but I chose the 6C45P because I use it in my own RC RIAA version of this Phono and it’s dead quiet! I personally use Cca Siemens tubes in the second stage but the sky is the limit here.

For simplicity the CCS could be substituted with a 12 – 15K R with a loss of approx of 2db. Alternatively, the purist may opt to throw in another 6C45P like I do. For those with an aversion to the CF stage try Allen Wright’s suped up version.

Comments Welcome.

Naz
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reVintage
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Post by reVintage »

Hi Naz,

Nice to see your approach.

LCR will not work with this much DC through the coils, but it´s an easy fix. Have also added the Tango DCRs. I have sold my Tangos and now have Lundahls that have slightly different DCR.

Your 4th time constant works like a charm, better phase response than the one I use 8) .

Think you will have to work a little more on your lowend though, as phase is off by 150 degrees. Hard to get around the 600ohm termination, isn´t it :? ?

Have also added my solution where the 600ohm termination resistor is at the input of the LCR.

I have changed the revRIAA to a corrected theoretical spice model proposed by Hagerman (fig 4 in his PDF). I mailed him as I found an error in it and he admitted the mistake.

To try out the driver you could do a RC-network with 600ohm series resistor.

Excited to see your further development!

EDIT: Just for fun I did a 1200 CR to compare with! Also changed to another second tube to get rid of excessive gain.
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Lars
Naz
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Post by Naz »

Hi Lars,

Thanks for your comments, much appreciated.

>>LCR will not work with this much DC through the coils, but it´s an easy fix<<

Silk implies that it will work with theirs. I didn't want to resort to a large DC blocking cap. If you have had experience with Tango LCRs can you advise hat max DC current they will handle please?

>>have sold my Tangos and now have Lundahls that have slightly different DCR<<

Why did you sell them and how do you like the Lundahls? Will they handle DC current?

I had considered using exactly the same R in series to get input to 600R but found little improvement in the Sims. Your comments re low end made me revisit and I've tweaked a few values to improve both low end and top end RIAA accuracy including adding back the 560R. Phase response is good down to LF where it doesn't really matter IME.

Thanks for the revised rev RIAA. I had some reservations about its absolute accuracy.

Rgds,
Naz
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reVintage
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Post by reVintage »

Hi again Naz,

Still I think one should forget about several mA through any coils :wink: . If you check my solution you should be aware of that it will probably not have none, whatsoever impact on soundquality.

Did some googling about Silk and nowhere is to be found they will accept current through them. What is said, is that they will tolerate some DC-voltage, saw a figure of 63V(10k-version?)! This indicates they don´t have Tangos 68k and 14k resistors bypassing the grounded caps. Also it seems like they are already endterminated with 600ohm.
Brgds
Lars
dave slagle
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Post by dave slagle »

In another thread here Jeepman mentioned that he strongly suspects the internals of the silk are in the picture below.

Image

There is a lot of confusion on the asylum and i have tried to correct things but it seems pointless.

It hs been stated that the use of a 600R resistor on the output of the silk will "ruin" the riaa. This is ONLY the case in the direct coupled version and the problem is the ~150ma of current the 600R load would draw through the LCR. In other words, the cautions against the 600R termination are only for situations where there is DC across the unit. The 100V limit for the DC is simply a function of the cap voltage rating.

The tango unit have additional resistors in parallel with the caps so they are not DC isolated and any current through them *may* cause problems.

Again my guess is that SILK chose their design based on the need for DC isolation so it could be used in a direct coupled circuit. The unit will drop in for the tango with only a slight loss in accuracy.

The whole 300R impedance is another point where people are missing it. The input impedance to the LCR is ~600 ohms independent of output termination. If you load the output with 1R, 600R or 100K the source still sees 600R. (thanks again to lars for pointing this out to me)

I will also add that the comments about DC through the inductors are also quite misleading. Just because some of the versions are not happy with DC through the inductors it doesn't mean that 80% nickel devices could not be designed to handle moderate amounts of DC current which means the Tango topology could easily be direct coupled with the proper choice of inductors.

Finally since this is my forum I feel comfortable saying that I do not know why anybody would want to buy a canned module and limit themselves to someone elses choice of compromise. I think I know where you can get a set of the inductors designed to your exact needs (and any desired network impedance) then you can choose (and experiment) with different types of caps and resistors and tune the overall accuracy in circuit.

dave
reVintage
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Post by reVintage »

Thanks Dave,

Think Silk and other uses permalloy. From what I heard these cores don´t like DC current, true?

Can understand your chokes with nickel cores will accept some current as they are both bigger and of other material.

Good point about buying a canned module with someone elses choice of compromise :twisted: !
Brgds
Lars
dave slagle
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Post by dave slagle »

Hey Lars,

It is possible to design the LCR inductors to handle DC even on an 80% nickel core. (my guess would be 30-40ma for the 600R values) It always bothers me when someone takes something a manufacturer says about their product and extends it to a general statement of fact about all products. The running of DC current through nickel inductors is a classic example of this.

dave
reVintage
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Post by reVintage »

Dave,

Thought we where talking really small cores here. Had the tiny Silk cups in mind that holds 2 chokes, at least two caps and probably 4 resistors. Neither the Lundahls or the Tangos(with two chokes in each) are that big......

To take the current you are talking about must ask for bigger cores and gapping?

The Tangos where said to handle 10V out, must mean 100V out.
Brgds
Lars
Naz
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Post by Naz »

[quote]Hi again Naz,

Still I think one should forget about several mA through any coils :wink: . If you check my solution you should be aware of that it will probably not have none, whatsoever impact on soundquality.

>Thanks Lars, I was trying to find a solution to make it easy for those who wanted to use commercial LCRs. Your mods work well on paper and with the right caps I would expect them to sound good too but they do rely on getting to nodes within the cans that I assume would not be accessable.

[quote]Did some googling about Silk and nowhere is to be found they will accept current through them. What is said, is that they will tolerate some DC-voltage, saw a figure of 63V(10k-version?)! This indicates they don´t have Tangos 68k and 14k resistors bypassing the grounded caps. Also it seems like they are already endterminated with 600ohm.

I think I'm over trying to get info on the commercial units. To my mind if the manufacturer isn't willing to provide sufficient info then why support them. I like Dave's idea of making the LCRs up, then we can do what we want.

BTW, what do you think of the minor tweaks?

Cheers,
Naz
dave slagle
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Post by dave slagle »

I use the same sized cores as I do for the autoformer volume controls for the units up to say 4K. Above that I go up a core size for the larger value choke.

dave
Naz
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Post by Naz »

Dave,

Thanks for the info, I concur with everything you stated. To my mind it's up to manufacturers to supply adequate technical information on their products and one shouldn't have to guess whether the LCR will accept DC current and at what level.

Originally I was only looking at helping Jeff but this has made me curious enough to mod my own Phono stage. I will advise Jeff to make the LCRs up if he still wants to attempt the project and I will do the same. As you said we have complete flexibility then.

Please email me if you are interested in making some inductors (for both of us) that will handle the small amount of DC current required by my circuit.

[Edit] Sorry, I'm not all that familiar with this forum and I think my replies are out of whack with the intended post.

Rgds,
Naz
dave slagle
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Post by dave slagle »

Hey naz,

The inductors to handle the current you require are not a problem at all. I also provide them with taps at around 3% to give the ability to tune the circuit. with a +-20% range you get a nice ability to tweak things. When i did the first 6K unit, I found that i actually needed about 5% more inductance that the sim suggested to get the best response. Luckily sweeping values in spice quickly shows you what direction you need to go in order tune things to get the desired measured response.

If you are going to go from scratch I'd also suggest ~730 ohms as your impedance since the cap values are much easier to source. (4uf and .1uf) I suspect the cap selection is one of the dominant "flavors" in this circuit and having the ability to try different types will net you a wide variety of tones.

dave
Naz
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Post by Naz »

Excellent!

Do you have a preferred circuit or do you suggest sticking with what we have? Before answering this one have you had a chance to check out the last tweaked ASC yet? If OK then can you supply the values to save me a bit of time ... I really don't have much of it at the moment!

Please email me with price, leadtime and any other relevant info.

Cheers,
Naz
Naz
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Post by Naz »

Dave,

Further thoughts, if you have already done a 6K version and you consider the performance to be in line with the 600 (730 ) ohm units , then this provides the opportunity to get rid of the CF.

Can you confirm and also whether you've done any other impedance please? Any other thoughts on different impedance LCRs ... perhaps 7K3?

Naz
reVintage
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Post by reVintage »

Hey Dave,

If we are talking real world: How big would the chokes for a 1.2kohm LCR be? DCR around 50/10ohm and capable of handling 20mA. Guess max signalvoltage through the network will be 10-20V.

The terminating resistor together with the coils could then double as cathode resistor in the driver CF.
Brgds
Lars
Andrew
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Post by Andrew »

Dave,

Further thoughts, if you have already done a 6K version and you consider the performance to be in line with the 600 (730 ) ohm units , then this provides the opportunity to get rid of the CF.

Can you confirm and also whether you've done any other impedance please? Any other thoughts on different impedance LCRs ... perhaps 7K3?

Naz
Dave did 1k5 inductors for me, they worked out very well indeed. In the thread on here you will find how the end result measured against the Spice model we designed.

-- Andrew
Last edited by Andrew on Thu Mar 12, 2009 2:37 pm, edited 1 time in total.
reVintage
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Post by reVintage »

Hi guys,

Had forgotten about Andrews 1,5ks.

Dave, do you have allowable current and DCRs for a pair of these critters?
Brgds
Lars
Naz
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Post by Naz »

Thanks Andrew, finally got around to reading the complete thread ... some very good info there!

Sorry Dave should have done that before asking about 7K3s.

Can anyone describe what they like about LCRs vs RC, be interesting to see if it matches my perception? Been on my list to try for a long time but never got round to it, mostly because I was very happy with what I'd been running.

Naz
Andrew
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Post by Andrew »

Thanks Andrew, finally got around to reading the complete thread ... some very good info there!

Sorry Dave should have done that before asking about 7K3s.

Can anyone describe what they like about LCRs vs RC, be interesting to see if it matches my perception? Been on my list to try for a long time but never got round to it, mostly because I was very happy with what I'd been running.

Naz
They seem to just get "out of the way" and just make music, very fluid.

My first version put some current across Dave's chokes, perhaps 5mA, perhaps a bit more, it was terminated with 1Meg. I was a little concerned by this, so my second version which was partly inspired by Lars, used extra caps in the DC paths to ground.

cheers,

-- Andrew
reVintage
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Post by reVintage »

Still wonder how a passive RC with 1,5k series resistor would hold up against a LCR? Probably better than a std highohm but not up to LCR. To lazy to test.....
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Lars
dave slagle
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Post by dave slagle »

Naz wrote:Dave

Can you confirm and also whether you've done any other impedance please? Any other thoughts on different impedance LCRs ... perhaps 7K3?

Naz
hey,

I have indeed done 7K3's and they worked quite nicely. a small amount of DC through the units is not a problem (~10ma iirc) The caps needed are .4u and .01u.

dave
reVintage
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Post by reVintage »

Had forgotten about Andrews 1,5ks.

Dave, do you have allowable current and DCRs for a pair of these critters?
Any ideas?
Brgds
Lars
dave slagle
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Post by dave slagle »

reVintage wrote:
Had forgotten about Andrews 1,5ks.

Dave, do you have allowable current and DCRs for a pair of these critters?
Any ideas?
From my notes I have the following for andrews chokes.

So you are in need of a pair of 4.7075 hy 150 ohm chokes and a pair of
.10875 hy 35 ohm chokes.

What Kind of DC current are you thinking of? I would expect 20-30ma to be possible.

I'd have to run the numbers on a specific design to get maximum voltages.

dave
Naz
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Post by Naz »

7K3 sounds like the go Dave, more advantages than disadvantages (I hope).

I think I could keep the current down to around 5mA. I'm assuming less compromise by keeping it low and also that allowing small DC current will not compromise sound quality compared to blocking it.

Naz
dave slagle
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Post by dave slagle »

hay naz,

I believe that the use of a cap to block that 5ma of DC will do substantially more sonic damage (add more sonic flavor?) than the compromises required to allow the inductors to function properly with the current.

dave
reVintage
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Post by reVintage »

And how do we get around the coupling-cap after the LCR :? ?
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Lars
Andrew
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Post by Andrew »

Still wonder how a passive RC with 1,5k series resistor would hold up against a LCR? Probably better than a std highohm but not up to LCR. To lazy to test.....
I designed a low-ish Z passive RC, nominal Z was 4k7, this has been built by a couple of friends. It used a C3g pentode as the driver loaded with 4k7, terminated with high Z. Both have heard the 1k5, one commented that LCR is the next level up.

I should have said, as it might not have been obvious, that the 1k5, in both incarnations used a 22nF silver mica for coupling after the LCR, so the chokes were floating; the only DC was via the resistors in parallel with the caps to ground.

Assuming the chokes can take the DC down via the caps to ground, couldn't you load the first stage with 7k3, then into the LCR and finally direct DC couple into the next stage with a 1 Meg doing both grid leak and termination of the LCR?
reVintage
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Post by reVintage »

This is how I would do :twisted: .

Threw in some sand as tubes aren´t at their best used as lowimpedance-driving CFs.

This JLH inspired buffer is totally "see through"! Wrong transistor-types in the schematic though. You could also use a Walt Jung-style deep Class-A biased diamond.

An output transformer or bifilar+sandbuffer should replace the output cap.
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Lars
dave slagle
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Post by dave slagle »

Andrew wrote: Assuming the chokes can take the DC down via the caps to ground, couldn't you load the first stage with 7k3, then into the LCR and finally direct DC couple into the next stage with a 1 Meg doing both grid leak and termination of the LCR?
I thought of that some time back, but remember the 7K3 load resistor || 7K3 LCR means a 3600 R load on the 6C45... Now before you go asking for a 14K6 LCR why not this....
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I didn't tweak it much but you get the idea.
I didn't tweak it much but you get the idea.
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reVintage
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Post by reVintage »

This is the Walt Jung sand driver.
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