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phono stage RIAA question.

220.236.97.102
Posted on February 25, 2011 at 20:24:13
lovetube
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I just playing around with this japanese DIY phono stage and found it is quite a good sounding unit but it have slightly bass shy compare to my other phono preamp . i hand draw the circuit so you guy can see what it is. they run only half of the 12AT7 and leave the other half doing nothings . any benefit to paralell this 12at7 ?
is there any way i can bass boot this phono stage ?
I already increase the coupling caps and the output cap .
cheers
LT

 

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RE: phono stage RIAA question., posted on February 25, 2011 at 22:21:11
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 It appears to me that the EQ you posted is lacking about 5dB of boost at 20Hz (relative to 1kHz). My suggestion is to search the Web for a more accurate RIAA network. There are quite a few variations available from reputable DIYers.







 

RE: phono stage RIAA question., posted on February 25, 2011 at 22:24:21
lovetube
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 Thanks .
will do.
LT

 

RE: phono stage RIAA question., posted on February 25, 2011 at 22:53:31
Triode_Kingdom
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Posts: 10042
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 I just took a second look at this, and the curve is worse than I realized. The ~17dB rolloff from 1K to 10K isn't there. I have to ask, are you sure you sketched it correctly? The network seems to be missing parts.

 

RE: phono stage RIAA question., posted on February 25, 2011 at 23:28:57
lovetube
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 yes my drawing is right . there no other parts there .
I have thought this RIAA wasn't right and hence the header of my post .

cheers
LT

 

RE: phono stage RIAA question., posted on February 26, 2011 at 02:36:56
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002
 You need to show the values of the cathode bypass caps. At low frequencies they will alter the valve's Rp and therefore the drive Z of the circuit to the EQ. You cannot look at the EQ in isolation and find something from elsewhere and pop it in.

Best way is to sim it in LTSPICE (which is so easy to learn) and derive the right values. You could learn it in as much time as it would take to mess about trying other methods .... unless someone did it for you of course ;-) You will also learn a lot that you can put to good use in the future.

Check out the link below.

cheers,

Stephen

 

The 2122 Hz pole cap is missing. But junk this schematic. nt, posted on February 26, 2011 at 03:23:20
kurt s
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Location: California
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RE: phono stage RIAA question., posted on February 26, 2011 at 03:37:19
Thomas Mayer
Manufacturer

Posts: 602
Joined: May 8, 2001
 Hi!

As others have mentioned already the RIAA circuit is not correct.

First thing to check with a new built is to have a look at the frequency response, especially with RIAA preamps. Just blindly changing component values is poking in the dark.

Even with a proven schematic the RIAA response should be checked and most often needs to be trimmed in circuit. In many cases miller capacitances and rp of driving tube are a substantial part of the RIAA response.

Also a good practice with checking a RIAA preamp is to first measure the frequency response of the circuit without the RIAA network. Replace it with a 1:10 voltage divider. If the frequency response is ok or as expected, put in the RIAA and recheck. This is best done by feeding the preamp through an inverse RIAA network from a signal generator.

Best regards


Thomas

 

Harsh but I would agree., posted on February 26, 2011 at 05:32:39
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002
 There are way better designs out there to play with.

cheers,

Stephen

 

RE: phono stage RIAA question., posted on February 26, 2011 at 07:27:52
grhughes
 Go to Steve Bench's audio pages and look at the RIAA preamp section, especially the 12ay7/6DJ8 cascode. The 12ay7 is cascaded for lower equivalent input noise. Ray

 

RE: phono stage RIAA question., posted on February 26, 2011 at 08:11:01
Mike C
Audiophile

Posts: 1074
Location: Essex
Joined: November 23, 2000
 First, the circuit error.
The 330k resistor going to earth should not be a resistor, it should be a capacitor of roughly 0.33 x the value of the other capacitor.
I.e. 0.33 x 0.01uF giving 0.0033 uF.

Even so, the values of 0.01uF and 0.0033uF do not look right for the 330kohm series resistor. But for the sake of a simple component change you could try it.

I strongly suggest you search for Kurt Strain's excellent post (several years ago) on configuring an riaa circuit; you will learn a *lot* from it. You will also see why others suggest you try a different circuit.

I agree with others that this is not a good circuit and I suggest the following reasons.

I do not like the 330kohm series resistor. Stage 1 will struggle to drive stage 2 with such a large resistance in the way.

I do not like such small caps in the RIAA. The miller capacitance of the 2nd valve, and any stray capacitance, are highly significant compared to these small caps. I would expect the circuit to be difficult to get right for this reason.

I hope this helps ...

 

Passive EQ phono stage RIAA question..., posted on February 26, 2011 at 09:05:21
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
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 ...can also be inspired by the comprehensive work at JE Labs. Check out Joe's website. He was inspired by the RCA passive EQ networks originally used with a 12AY7, but adapted and tweaked the values for the 6SL7. Your network can be reworked this afternoon if you have the parts on hand...

 

Passive EQ phono stage RIAA question., posted on February 26, 2011 at 17:21:24
Interstage Tranny
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Posts: 3063
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 Your circuit has a slight error, causing the lack of bass boost. You have a 330K resistor across the 33K resistor. Change that 330K R to a 3300 pf cap for a quick fix. The resistors can be tweaked, as well, but, this 3300 pf cap will quickly yield the required bass...

Concerning the required RIAA treble deemphasis, if your circuit does not satisfy your ears and you want more high frequency cut, you can add another EQ filter pole to ground after this network.

Research the passive phono EQ published circuits beginning circa early 1950s. RIAA was standardized in 1954. Many, many passive EQ circuits have been implemented with 6SL7s since then. Many are available online. In my lower post to this thread, I gave the link to the JE Labs site. Joe's mono playback system has many virtues. His design topology can easily be implemented as dual mono stereo preamp stages. By using separate power supplies for each stage as well as each channel, ultimate detail and stereo separation with amazing soundstage effects can be achieved.

While I am a bigger fan of active feedback phono EQ, made popular in the early stereo era, I do know more than a few folks that have implemented the passive EQ to their satisfaction. Happy tunes await you; so, ENJOY !

 

Thanks IT and other who response ., posted on February 26, 2011 at 17:30:31
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 I will have to learn a bit more on this RIAA stuff.
look like it is going to be complicated to get right.
Cheers
LT

 

Hi Interstage Tranny, posted on February 26, 2011 at 17:37:20
lovetube
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 isn't the 330k is the grid leak for the 6CG7 ?

 

Hi ! It's not difficult at all; just tedious work; after reading over and over..., posted on February 26, 2011 at 18:00:00
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
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 It can take a long time for the light bulb to be fully lit concerning electronics. Sometimes, the technical theory simply requires memory to "move on" and understand further. As a great example, current flow theory is often taught as passing from positive to negative, or vice versa. In order to work with high current carrying voltages safely, understanding the flow is much less important than remembering not to get any part of your body in the way of any possible current flow, in order to reduce shocks...
You are very welcome !


With EQ circuits, only via studying and rereading, again and again, will the filtering/boosting be comprehended. Seeing a wide variety of circuits, they actually start looking similar. While many peers might believe tweaking cap and R values is confusing, I believe that if you can trust your ears, simple value changing is extremely enlightening. In fact, simply changing the type of EQ capacitor can be ear opening; say ceramic to silver mica or polystyrene.


You are correct that the second 330K to ground is the 6CG7 grid leak R. In many passive preamps, this grid leak value is even higher. For instant satisfaction, simply install a 3300 pf across that 33K R in each channel's EQ pole. Then, later you can upgrade or change at will, after a bit of research. I assure you that the many circuits really do start appearing similar. Remember that the type of EQ cap installed or reinstalled will probably startle your ears. After a bit of break-in time for new parts, your ears will easily tell you whether that circuit prefers silver mica or polystyrene caps. Enjoy the ride and happy listening...

 

Google: "Jeremy Epstein 6C45P Phono Preamp page", posted on February 26, 2011 at 20:36:09
kurt s
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Posts: 1137
Location: California
Joined: October 12, 2009
 It has calculations needed. I know the dude took info I provided for free, and then writes on his page:

"©2003, 2004 Jeremy Epstein. No reproduction, retransmission or web distibution without prior written permission."

Thanks, Jeremy, for your lack of citations and your illegal claim to having no one do "web distibution [sic]". It's already published to the web! Jeez.

 

Thanks Kurt but i use the KAB compute for RIAA value, posted on February 26, 2011 at 20:42:31
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 not sure how it will sound yet.
working on RIAA is funs but finding the correct components is a pain in the butt...

Cheers
LT

 

RE: Hi ! It's not difficult at all; just tedious work; after reading over and over..., posted on February 26, 2011 at 20:45:01
lovetube
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 Thanks.
will have a lot of trial that is for sure but you are right , i always truth my ears .

 

RE: Thanks Kurt but i use the KAB compute for RIAA value, posted on February 26, 2011 at 23:14:10
kurt s
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Posts: 1137
Location: California
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 "working on RIAA is funs but finding the correct components is a pain in the butt..."


Well, in a phono RIAA, the series resistors and coupling caps have greatest impact on the sound. You may want to use 0.01 uF and 0.027uF + 0.003 uF for the 2.916 exact ratio of capacitors (<3% error at factor of 3.0), as they are available in high quality caps. 0.003 uF, or 3000 pF might be best to go to mica. For resistors, use same kinds but allow yourself some slack or trim with paralleling or series-connecting resistors.

Good luck.

 

KAB Lipschitz RIAA calulator, posted on February 26, 2011 at 23:48:59
kurt s
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Posts: 1137
Location: California
Joined: October 12, 2009
 Here's some tips: Start trying it with R1 = 220,000 ohms. Then many typical values will be close for you.

Correction to my other post: 0.01 uF / 2.916 = 0.0034 uF
= close to 0.33 uF.

Lipschitz' formula here expects a zero output impedance from the op amp. 220,000 ohms is the total source impedance including Rp and Rl in parallel at first tube's plate. (Rp || Rl) + R1 = 220K.

If you direct couple this, expect a weaker bass, as V2 grid current will affect the small signal's DC effect toward it.

If you choose to AC couple this, the bass becomes more accurate and can even make it sound better. The grid current then goes out of play. And the coupling cap quality becomes very critical.

If you choose to AC couple and need a grid grounding resistor at V2, then make it as high as possible because this calculator expects the load there to be infinite. This is where very high grid resistor values and grid leak biasing can get real close to that on V2.

You will never achieve a perfect RIAA. :-)

 

RE: KAB Lipschitz RIAA calulator, posted on February 27, 2011 at 00:45:17
lovetube
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 Hi kurt.
I did that already last night with the R1 of 220k and come up with r2= 31.989K and the C2 =0.0034uf and c1 = 0.009940uf.
so i'm not sure why your value is different ?
I'm AC coupled the first to the second stage /. however as to my drawing I think is might be best to parallel the 12AT7 for lower impedance .
what you rekon ?
Cheers
LT

 

I can't do math anymore, maybe :-), posted on February 27, 2011 at 01:58:16
kurt s
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Posts: 1137
Location: California
Joined: October 12, 2009
 Your results are correct from the calculator.

Where I went wrong was thinking C2/C1 = 2.916, and still missed; But in reality, C1/C2 = 2.916,

or C2 = C1 / 2.916.

C1 = 0.01 uF; therefore C2 = 0.01 uF / 2.916 = 0.0034 uF. That's the correct answer. Thanks for catching that.

 

RE: Thanks Kurt but i use the KAB compute for RIAA value, posted on February 27, 2011 at 11:22:33
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002
 The KAB calculator will get you close but not the right answer.

It assumes the source impedance is very very low and in most valve circuits it isn't. It also assumes the load which is the grid bias resistor of the 2nd stage is 50 times that of the source resistor. If these two conditions aren't met, the calculator will be off. Depends on how accurate you want to get this thing. Say your series R is 330k, then 50 x 330k = 16.5M for the 2nd stage grid. As Kurt days, grid leak bias can be handy and get you close but not all valves like it and that's not your circuit at present.

IME, the best way is to sim it. Don't bother messing around with calculations unless you know all the variables and how they interact. The EQ loads the first stage but not in a linear fashion as it's impedance changes with frequency. With high Z EQ networks, the Miller capacitance of the 2nd stage will affect the values.

You could measure it but you will go nuts trying to dial in an all-in-one EQ this way. A sim can get you there in very little time. I use them all the time to determine EQ values. Before sims were easy, I had a mammoth spreadsheet that would do it but even that was hard work and error prone. A few sums won't get you there very easily.

cheers,

Stephen

 

Hi Stephen can you help me sim my circuit please, posted on February 27, 2011 at 15:26:36
lovetube
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Posts: 3008
Location: Melbourne Australia
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 Hi Stephen.
Thanks . yes I see what you means . i did the KAB calculation and run it on the pre but when I scope the frequency response I now see the bass boost way down to under 10hz but the top end is off . it wouldn't pass 5khz without dropping 5~7db .
so your post definitely explain it quite well .
I have download the sim program from linear.com which you send me the link of , but haven't extract it yet as it seem to requires a different program than what i have to open it .
for now can you help me to sim the circuit on your program ? see how this circuit come up .
the circuit running one section of the 12AT7 first stage and 6CG7 in Parallel on second stage. the series R was changed to 220k and all the caps is as my other post to Kurt.
Thanks
LT

 

not to worry kurt , we all do that some time, posted on February 27, 2011 at 20:50:04
lovetube
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 Cheers
LT

 

There you go, posted on February 28, 2011 at 05:38:03
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002


See links for sim of something like your RIAA. It's not totally accurate as I don't know the value of the cathode bypass caps or the cathode resistor for the 6cg7 so I used the sim to work out a value.

It sims 20 - 20k within a 0.08dB window. Use 1% parts and it will be close to +/- 0.1dB or so. You'd need very expensive test equipment and a meticulous setup to verify any of this to that accuracy IMHO.

If you want to take it further, download the .asc file and play around with it in LTSPICE. There's a lot you can learn by doing this that will be invaluable for the future. You'll need the drop down valve models from the Intact Audio site, Sim section.

cheers,

Stephen

 

Thanks Stephen, posted on February 28, 2011 at 05:58:31
lovetube
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 is the C2 is 40n right ? I can't see it clear enough .and R11 is 1000K not 100k right ?
the cathode on the 12AT7 is 1K at prerent .but the cathode on the 6CG7 is 2.7k .
I thinking of paralell the 12AT7 as well . what do you think ? it should have lower the impredance of the stage right ?
Cheers
LT

 

RE: Thanks Stephen, posted on February 28, 2011 at 07:26:10
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002
 C2 is 10n. R11 is 1000k or 1M.

2.7k seems a bit high according to the sim for a paralleled 6CG7. What anode voltage does it run at? I had to guess an HT as well and used 300V. With 2.7k, I'd expect the anode to be about 260V on a 300V HT. Still sims OK. Maybe it sounds better that way? It's not a valve I have any experience with.

Paralleling the 12AT7 would lower the drive Z. It would change the series resistor to the EQ as well. I'm not a fan of paralleling things up. My preference is to chose the right thing in the first place.

cheers,

Stephen

 

RE: There you go...Thanks for your work and Sim..., posted on February 28, 2011 at 20:22:31
Minussss3db
Manufacturer

Posts: 586
Location: so cal
Joined: January 28, 2010
 Sincerely,\

this gives a lot to play with in one package...

sincerely,
-3db

 

RE: Google: "Jeremy Epstein 6C45P Phono Preamp page", posted on February 18, 2017 at 13:00:15
J Epstein
Reviewer

Posts: 1041
Joined: April 6, 2000
 Dear me.

I'm just noticing this post, 6 years later.

I'm not sure who Kurt S. is or where he published his RIAA phono calculations, and since it's been 14 years since I wrote that article, I can't conclusively recall every source I used in gathering the necessary calculations, SEVEN YEARS before Kurt S. complained about my plagiarism. But it should be pretty clear that my main source was Morgan Jones, "Valve Amplifiers" which is cited at the bottom of the article.

I had no idea that a guy named Kurt S. had invented mathematics! Sorry dude, I'll be sure to cite you next time I add, subtract, divide or multiply anything.

I know full well I am standing on the shoulders of giants. I guess I just don't know all of their names.

2017 update: don't build the power supply exactly as it was shown in the "Rev 4" schematic: it will give you a low frequency "motorboat" oscillation. Every time. I have removed that part of the schematic. The lower image (main power supply drawing) is still Rev 4, that doesn't need changes.

I updated the power supply filter part of the main schematic up to Rev 5 to reflect this change, it's caused people pain every now and then for the past 14 years.

But they always like the way it sounds once they get it sorted out!

 

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