How to make an 600 ohm LCR have Zin=1200ohm

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reVintage
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How to make an 600 ohm LCR have Zin=1200ohm

Post by reVintage »

This is my first visit to this interesting forum. Nice to read serious discussions about LCR-RIAAs.

I bought Tango EQ-2Ls some 15 years ago, but gave up as I did not find the right driver for them. Decided to give them another try now that I had Spice.

Anyway, my solution is simple:
If you drive the LCR with low Z(White-follower) you can place a resistor of 600 ohm(minus the Zout of the driver) in series with the LCR to get almost the double Zin without signal loss.

It works IRL but I have not built the output stage. In Spice environment the frequency response at the output of the LCR is within +/-0,015dB with the original Tango component values. I have not done any measurements on the real circuit yet.

Unfortunately I have not had the opportunity to compare with a traditional LCR without series resistor and with 600 ohm terminantion at the output.
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dave slagle
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Re: How to make an 600 ohm LCR have Zin=1200ohm

Post by dave slagle »

hey lars,

Welcome to this little crazy corner of the audio world.

I'm not sure i follow your "1200 ohm Zin" you mention that the series resistor is 600 ohms - the Zout of the white follower which i presume is 26 ohms. Doesn't this simply mean that the LCR sees a precisely defined source Z of 600 ohms and therefore behaves as expected.

The sims seem to confirm this.

This approach is similar (in general concept) to those that use a pentode with a 600 ohm load resistor and unterminated secondary to provide a 600 ohm source. I actually think your approach of using a 25 ohm source with an additional series resistor to create your 600 ohm source is pretty clever. I'm looking forward to hearing how it sounds.

Makes me wonder about using something like a 6C45 into a 10:1 stepdown with a 585 ohm series resistor (subtract the copper values of the 10:1 from the 585) as another defined source method.

thanks for the ideas!

dave
reVintage
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Re: How to make an 600 ohm LCR have Zin=1200ohm

Post by reVintage »

dave slagle wrote: I'm not sure i follow your "1200 ohm Zin" you mention that the series resistor is 600 ohms - the Zout of the white follower which i presume is 26 ohms. Doesn't this simply mean that the LCR sees a precisely defined source Z of 600 ohms and therefore behaves as expected.
dave
Thanks for the input!

If we feed an unterminated 600ohm LCR with 600ohm in series from a source with neglible Zout, we get Zin=1200ohm without signal-loss. I can not explain why it behaves like this, but this is what my spice-calculations show, see below. Green is with termination, brown is with 600ohm serial resitor(Zin 1,2k).

The only difference I have found is that for the serial version the impedance is going down to 1,1k below 100Hz and from 50Hz and downwards is increasing to 1,8k at 20Hz. The terminated one is constant at 600ohm.

As I have not yet measured IRL I still hope the Spice results are true. I will of course comment on the sound when the output stage is built.

One could also use a 6C45 CF with a choke-load that has almost the same Zout as the 6N30-White.

If we use a 1,5k LCR, as mentioned in another thread, maybe a 6C45(if we assume Zout=1k) could do the trick through a 3,3:1 stepdown (100ohm out) with a 1,4k series resistor minus copper values?
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Post by dave slagle »

If we feed an unterminated 600ohm LCR with 600ohm in series from a source with neglible Zout, we get Zin=1200ohm without signal-loss.
Maybe it is just a semantics thing, but I would just say that this situation has a 600 source driving a filter that needs either a 600 ohm source or a 600 ohm load to behave as expected.

rather than a cathode follower, the 6C45 through a 10:1 will still have a gain of 4 and every little it helps. I'd say a 4:1 should work, but doing a nice wide bandwidth 4:1 is much harder to do so maybe the best tradeoff falls somewhere in the 6:1 ratio.

the thing that really appeals to me about the defined source is how easy it becomes to direct couple the two stages.

rather than a 1K5 lcr, if we go for the 7K value then a plate choke loaded 6C45 could be direct coupled through a 6K series resistor.

dave
reVintage
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Post by reVintage »

Maybe it is just a semantics thing, but I would just say that this situation has a 600 source driving a filter that needs either a 600 ohm source or a 600 ohm load to behave as expected.

"6C45 through a 10:1 will still have a gain of 4"

"rather than a 1K5 lcr, if we go for the 7K value then a plate choke loaded 6C45 could be direct coupled through a 6K series resistor"

dave
Still I prefer to look at it as a ca 1,2k input, as this is what we have to consider when designing the driver :wink: . But as you say it might be a "semantic´s thing". Or maybe "schematics thing" :lol: :lol: .

If I read between the lines I think it says: CF is a no-no! True?

As I see it, it is doable with a low Zoutput driver feeding 1,2k without the need of a transformer(or cap). The Loesch design with a E810F driving 300ohm(Ra//LCR) might be a clever idea but is not acceptable (overloadmargin,gain&distorsion). Anyway I must thank Mr. Loesch as it was after seeing his schematic I got the idea.
With the help of some clever thinking one could maybe eliminate the input cap of next stage, too.


In my design considerations I opted for a first stage gain of between 25-34dB at the LCR input, with an overload margin of at least 300mV and a total gain of 34-40dB.

From my point of view a first stage gain of 12dB is to low.

If we go for the 7k LCR with a 6k serial(Zin=13k), I think we might loose it, as my idea of why LCRs sound better is that we have small serial resistors. But I really like the idea of just a platechoke though.

What is the lowest DCR you can get if you make me a platechoke with 200-300H/10mA?

As a transformer novice I thought it was easier to make a good 1:1 or 4:1 than a 10:1, but you are the Tranny-Man!
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reVintage
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Post by reVintage »

reVintage wrote:
If we go for the 7k LCR with a 6k serial(Zin=13k), I think we might loose it, as my idea of why LCRs sound better is that we have small serial resistors. But I really like the idea of just a platechoke though.
The underscored from last post should of course read: "low series resistance"
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Andrew
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Post by Andrew »

Hi Lars,

I would underscore Dave here, a pentode, or a cascode, or triode into a step down is to me the best way I have found to go, that way you get both gain and a matched impedance for the LCR. I have contemplated a cathode follower driven RIAA but there were too many stages and it was too noisy (self noise)

In detail, my preference would be either E810F pentode, 6C45 cascode or high gm high mu triode, say 5842/6C45 triode via a step down, in no particular order.

If you select a higher Z network say 1k5 then more pentodes become available, I use a D3A, and more gain can be had from the cascode, a 5842 cascode could be good here now.

A 6k RIAA and the world open's up, but I imagine winding the chokes is more challenging, Dave is the expert, however.

I have spent quite some time over the last few weeks measuring Dave's chokes once they were in my modules (sorry Dave been meaning to publish this) suffice to say, I am very pleased with the 1k5's. Currently, I can detect no non-linearity with amplitudes.

I am close to getting a D3A into 1k5 running, second stage either a CCS loaded D3A or John Brokies Aikido to drive a TVC.

I will post more when I have sorted the regulated PSU's which have been a real pain.

cheers,

-- Andrew
reVintage
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Post by reVintage »

Hi Andrew,
I think you have misunderstood my "twist" on the LCR. To get the higher input impedance that I suggest, you have to feed the circuit with a low impedance. This means pentodes and cascodes are out of the question.

This leaves us with high-Gm triodes (or triode connected pentodes as E810F) with low to medium gain as Zout is mu/gm. For example a 6C45 is around 1k and a triodeconnected E810F is some hundred ohms lower. So if we connect "my" LCR-circuit to a triode gain-stage it must either be by a stepdown.

If we use a CF, Zout is 1/gm and we can skip the stepdown. The CF does not add any significant noise. But it could colour the sound as all components more or less do.
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Post by Andrew »

Hi Lars

I do understand your take, and it will certainly work, will it sound better than a cascode, pentode triode plus step down (which could be parafeed or traditonal)? Well I don't know.

This is what Thorsten Loesch did with a pentode.
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reVintage
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Post by reVintage »

Hi Andrew,
Interesting you youshould mention the Loesch-design :) ! I made a comment about it in my answer to David just before your post.
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Post by Andrew »

He did two, have you seen them both?

The first one was a high R load on the E810F into a very large cap, then the RIAA which was terminated with a 600R followed be a D3A common cathode second stage.

The second one, as above, was a 600R loaded pentode into the RIAA terminated by high Z, which was then cap-coupled into a grid leak biased D3A. So he made the coupling cap work for him by using it, and the high grid leak R, to grid leak bias the D3A thus reducing the D3A's Zout; rather clever really.

The white cathode follower idea will work, of course, but it is important to consider there's 3 times as many valves in that setup when compared to the pentode, so possibly 3 times as much self noise (hiss). I have no idea if that will be a problem, of course? I had a similar idea using an Aikido to drive the LCR but gave up because I felt a pentode or cascode would be better. That is not meant to be crtical of your proposal as it is the solution adopted by others and I would bery much like to hear it, about it. Have you seen this?
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Post by reVintage »

Andrew,
About Loesch I meant the second, rather clever, but in my eyes doubtful design as I said earlier.

Thanks for the schematic, interesting to see someone else use the White and the "series resistor-twist" before me. Thought I was innovative but this shows it was yesterdays news :( . How did you find it? Have searched a lot and never seen it.

The rest of the design does not look that promising though. Overworked but the thinking is to my liking! 4mA it not enough to drive the 1070ohm load and the gain is to high in the first stage where the pentode would work better into a resistive load. A pentode or cascode in the first stage will work but you should have a gain that allows an ample overload-margin(+300mV).
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Post by Andrew »

I wouldn't be disappointed, I have found there's very little in the valve world that has not be done before. Several times I have thought 'ahh that's new', only to discover many have been there before me. I see these events as re-enforcement of my thinking.

I would agree that the Loesch design could be considered to have a poor overload margin, I haven't built it so I can't comment on reality. I do know for a fact the distortion measurements of the first design are very good, so there is little at issue with the use of a pentode for the input. I cannot say for his second (mk2) as I don't know anyone who has built it, tho' I assume Throsten did measure it.

I'm definitely interested in hearing more why you feel the mk2 "might be a clever idea but is not acceptable (overloadmargin,gain&distorsion)".


PS lots of LCR phono's on Thorsten's Yahoo Group.
http://tech.groups.yahoo.com/group/Thun ... -LCR-RIAA/
all the best,

-- Andrew
reVintage
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Post by reVintage »

Is the guy who did the Mk2 some kind of holy man? Seems you should not question his circuits without getting lots of questions thrown back at you :( .

Anyway, all the shortcomings in Mk2 is so obvious. There is no preamp-tube in this world who likes 300ohms load :) .
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Post by dave slagle »

hey guys.

yes i agree that a 1:1 is easier than a step-down as long as you can do bifilar, when you get into sectional winds, the larger the stepdown the easier things get.

I have wound and tested 7K5 units and they work really well. The nice thing about them is the caps are .02 and .001uf really easy to source.

dave
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Post by Andrew »

Is the guy who did the Mk2 some kind of holy man?
No, not at all, but he has built one, no two LCRs, which as far as I know both work, this is no mean feat. I offer him due respect as one who has been there and, moreover, was willing to publish his work so we could all benefit from it.
Anyway, all the shortcomings in Mk2 is so obvious. There is no preamp-tube in this world who likes 300ohms load :) .
Forgive me, I'm a bit slow, I can see some flaws in this design but I am always open to further education, I was merely asking you to explain to me, in more detail, where these problems lie. I am building one of these phonos and if I'm going wrong it would be good to know where I have missed something.

Please be more explict, I think I can see what you mean by poor overload due to the potential lack of headroom at the pentode's cathode that is a consequence of the chosen biasing scheme. Is this what you meant?

But where is the poor distortion coming from? And, what do you mean by lack of gain? For example, what would you calculate the gain of the pentode stage to be?

thanks,

-- Andrew
reVintage
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Post by reVintage »

Hi Andrew,
Did not mean to be rude :oops: , but I read your earlier posts and was sure you had the insight of the problems.
About the Mk2:
1. Zero bias makes the overload-margin extremely low. Checked the UaIa curve and it shows some pecularities below 0V Ug. 50-100mV clip is not OK! Measurements made with Shure V15, many years ago, showed peaks of over 2V peak-peak. This is ca 0,7Vrms so we hope this record is not in our collection. Anyway, except for going for a high ol-margin we should also make an amp with good clipping behaviour.
2. The E810F works into a load of 300ohm. Pentode-gain is calculated as G=gm*Ra//Rload and this is about 15x. Too low!
3. I am sure working into 300ohm will also be a source of bad linearity.
4. With these imperfections a circuit of this kind might sound charming but it will just be a charming coloration.
So this was my concerns about the Mk2.
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reVintage
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Post by reVintage »

Dave,
Saw that you made a 3:1 tranny for sbenchs LR. Couldn´t one of those be used together with a 1,5k LCR? Or maybe a triode connected E810F (Ri 700ohm) together with a 3-400ohm series resistor and a 600ohm LCR?
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Post by Andrew »

Hi Andrew,
Did not mean to be rude :oops: , but I read your earlier posts and was sure you had the insight of the problems.
About the Mk2:
1. Zero bias makes the overload-margin extremely low. Checked the UaIa curve and it shows some pecularities below 0V Ug. 50-100mV clip is not OK! Measurements made with Shure V15, many years ago, showed peaks of over 2V peak-peak. This is ca 0,7Vrms so we hope this record is not in our collection. Anyway, except for going for a high ol-margin we should also make an amp with good clipping behaviour.
2. The E810F works into a load of 300ohm. Pentode-gain is calculated as G=gm*Ra//Rload and this is about 15x. Too low!
3. I am sure working into 300ohm will also be a source of bad linearity.
4. With these imperfections a circuit of this kind might sound charming but it will just be a charming coloration.
So this was my concerns about the Mk2.
Well, true insight, no, I'm a mere journeyman.

I agree (agreed) about zero bias, as we said, this is a very very valid point, I do know why Throsten did this, but I cannot reconcile his choice. In my opinion, given he uses a step up transformers, fixed bias would have been a better choice (assuming the txs can live with a little dc) there's also battery bias, diodes and LEDs etc and a, finally, big cap. To me LEDs look good as does fixed bias.

I'm a bit confused about where the 300R load is? I see the 600R (604R) on the pentode? As I understand it, Zout of a pentode is Rp in parallel with Rload so that's 604R parallel with a ~100k giving about 600R Zout. So the tube offers a 600R load to the RIAA and a gain of 600 * gm which will be a bit less than 50, so realistically a little under 30 or around 30db. Tho' perhaps my understanding is flawed?

-- Andrew
reVintage
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Post by reVintage »

Yes, I agree a LED seems to be a good choice, must try it. But also heard some of them are noisy.

About the load and Ra. Ra is as you say ca 600 ohm but Rload is the 600ohms LCR. So therefore the pentode is looking into 300ohms. And 0,3*50=15, can not see it any other way. This means you have 24dB from the E810F and the loss in the LCR is -17dB. The D3a has a measured gain of just over 32dB with 8k load according to P. Millett. All together this makes a total gain of just below 40dB. With most of the gain after the LCR this indicates a noisy circuit as the signal coming from the LCR is in the region of 10-12mV!

If we instead have a gain of 32dB in the first step and 24dB in the second you win a lot as the noise from the first stage is filtered through the RIAA.
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Post by dave slagle »

reVintage wrote: About the load and Ra. Ra is as you say ca 600 ohm but Rload is the 600ohms LCR.
has anyone calculated the load vs. frequency of one of these units? do they indeed provide a 600R constant impedance independent of frequency?

just curious.

dave
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Post by Andrew »


About the load and Ra. Ra is as you say ca 600 ohm but Rload is the 600ohms LCR. So therefore the pentode is looking into 300ohms. .
Looking at the schematic, isn't it terminated with 1meg5? Perhaps I'm missing something?

cheers,

-- Andrew
reVintage
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Post by reVintage »

Hi Dave,
Here are the spiced impedance-curves that you ask for. One with 600ohm termination and the other with 600ohm series at the input. As you see the traditional is close to 600ohm. The seriesconnected is loading the source less at lower frequencies, lowest is 1,1k at 85Hz below that increasing to 1,9k at 20Hz.

Fed from a 0 ohm generator they have close to identical frequency- and phase-response from 10Hz to 1MHz. The terminated one has the same response independent of the generators Ri. The series connected must have Ri+Rseries=600ohm.

OT: Dave could you please check my post under Transformers about the current-cancelling 1:1 driver, I am curious if it even will work!
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reVintage
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Post by reVintage »

Andrew wrote:Looking at the schematic, isn't it terminated with 1meg5? Perhaps I'm missing something?

cheers,

-- Andrew
Hi Andrew,
Yes, the LCR is terminated with 1,5M but the LCR it self is loading the driver with 0,6k. Check the impedance curves in the post above.
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Post by Andrew »

Hi Lars,

Would you post the schem's for both tests, without them I cannot see what mean? Perhaps my circuit is wrong?

many thanks,

-- Andrew
reVintage
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Post by reVintage »

Andrew wrote:Hi Lars,

Would you post the schem's for both tests, without them I cannot see what mean? Perhaps my circuit is wrong?

many thanks,

-- Andrew
Why not post the schematic of your tweaked 1,5k? Can do the the impedance check on those instead.

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Post by Andrew »

About the load and Ra. Ra is as you say ca 600 ohm but Rload is the 600ohms LCR. So therefore the pentode is looking into 300ohms. And 0,3*50=15, can not see it any other way.
OK, I did a review of my theory, yes its a 300R load, I forgot to || in the RIAA's 600R to ground. So, you are quite correct, seems the second circuit has two potential problems then.

However, with a 1k5 and E810F the load is much higher, 750R in total, so much closer to your stated goal of 32db from the E810F and if using LED bias, or fixed bias etc then the overload problem is also reduced.

thanks for the help,

-- Andrew
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Post by Andrew »


has anyone calculated the load vs. frequency of one of these units? do they indeed provide a 600R constant impedance independent of frequency?

just curious.

dave
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Post by dave slagle »

hey guys,

thanks for the Z plots, i now know how to do it in spice (forgot you could enter an expression)

i never realized that the unterminated LCR still provided roughly the same load to the driver.

thanks for the education.

dave
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Post by Andrew »

Well, I should thank Lars as well, he made be re-evaluate my understanding of the pentode input circuit, turns out the gut feel for a slightly higher Z was justified but its good have the theory correct in my head.

cheers and thanks,

-- Andrew
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