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Old 9th Jun 2019, 9:58 pm   #21
Radio Wrangler
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Default Re: Valve preamp.

Quote:
By the way, something I haven’t seen – although it might well exist – is a scientific/engineering rationale in support of passive RIAA equalization. On its face it seems like another exercise in doing it the hard way, when by using a series NFB loop one neatly disposes of the initial gain and equalization requirements in one fell swoop.
I haven't seen any proper argument in its favour, just unsubstantiated statements by pundits.

It's odd that feedback volume controls like the little Cambridge Audio amps are seen as laudable and their effects on dynamic range, noise and headroom are trumpeted.

But the same advantages of applying RIAA equalisation in a feedback network are pooh-poohed.

We aren't in Kansas anymore!

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Old 9th Jun 2019, 10:41 pm   #22
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Default Re: Valve preamp.

There is every flavour of RIAA topology. Either fully in the feedback loop, either around the input stage or after an initial flat gain block. Or a mixture of feedback and passive. Or entirely passive, possible split with a buffer in between.

I can find no rationale for going passive. Active RIAA around the input gain stage definitely wins with S/N ratio and overload margin. However, having tried all sorts of EQ arrangements, I have ended up with passive. For some reason it sounds more musical. And at normal listening levels you need to put your ear almost at the speaker to hear any hiss (on a moving coil cartridge with 300uV at 5cm/sec output)

Daft isn't it? But there it is.

Craig
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Old 9th Jun 2019, 11:42 pm   #23
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Default Re: Valve preamp.

Its interesting to see one of my designs posted here
https://www.vintage-radio.net/forum/...d.php?t=102751
Even posted by Andy in his quest for a phono stage. The second design in the link was specifically designed for a US client. hence the transformer in and out. The 6BR7 is slightly better on noise than the EF86, but even better is the CV4006.
(6BR7 and CV4006 are the same pinout, but are different to EF86)

The 6DE7 is a dissimilar dual triode. The first triode is roughly equivalent to a 12AX7, and the second stage is roughly similar to a 12B4. Its design was originally for the vertical oscillator and vertical output for B/W TV receivers. I used it because it has the gain required to re amplify the output from the pentode after RIAA correction, and the current drive to work the output transformer.

The input stage using the CV4006 followed by a 12AX7 is the phono stage I listen to every day, and is incorporated into my little 7 watt/channel 6BW6 amplifier.

The phono stage ( to be an audiophool) is extremely smooth and pleasant to listen to.
Its DC heated, but incorporating it into the power amp box, even with a thick steel screen between the phono stage and output stage + power supply is NOT the quietest way to do it. At full volume I do have a little hum but its inaudible at "normal" listening levels.

Joe
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Old 10th Jun 2019, 5:34 pm   #24
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Default Re: Valve preamp.

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Originally Posted by Craig Sawyers View Post
Here's some modelling of Audio Research's capacitor with inductor etc.

Of course David is right. The best result is for just the electrolytic alone. The attached shows the comparison. I have chosen the values that Audio Research used in the LS22 - 6,600uF (3 x 2,200 in parallel). The vertical axis is the same in all three cases for direct comparison.

Craig
The results look odd, did you include the 0.22uF?
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Old 10th Jun 2019, 6:24 pm   #25
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Default Re: Valve preamp.

Quote:
Originally Posted by PJL View Post

The results look odd, did you include the 0.22uF?
Aha - well spotted. I modelled with 2uF! I'll re-run the simulation....


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Old 10th Jun 2019, 6:34 pm   #26
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Default Re: Valve preamp.

Basically the same shape but with the resonant peak moved to a higher frequency. Which is not at all surprising really.

Craig
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File Type: pdf With 220nF.pdf (86.0 KB, 18 views)
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Old 15th Jun 2019, 3:06 am   #27
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Default Re: Valve preamp.

Seems this problem has come up before!.
An article from 1951, WITH some solutions from that time.
Joe
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File Type: pdf Audio-1951-Jun1.pdf (317.9 KB, 16 views)
File Type: pdf Audio-1951-Jun.pdf (342.7 KB, 18 views)
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Old 15th Jun 2019, 6:45 am   #28
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Default Re: Valve preamp.

I've built one phono circuit to test so far, IE the Valve Wizard ECC83 based jobbie. I built this using Tektronix ceramic wafers, I can get the whole circuit on two 11 "pole" wafers. It seems to work ok however I have 50hz modulating the OP. With the circuit knocked together on an old chassis 50hz was at about 60mV P-P, I reduced this to 20mV P-P by lifting the ground and reduced it a bit more by moving the circuit away from the mains tfmr/PSU section and putting it in a shielded steel box, it's still too much though.

Another issue I've hit is testing said phono stage using an inverse RIAA filter. I built this one - http://sound.whsites.net/project80.htm the signal in is really attenuated, but after some thought I suspect this to be normal. I thought at first I could IP a 2mV sine, a typical cart OP but there's nowt comes out, obviously in retrospect.

I'm a little uncertain about R3's value, there are a few inverse RIAA filters I've found, all use the same R's and C's more or less, EG 833k, 75k, 3n6 and 1n, but use different value's for R3, anywhere from 600r to 3k3. More testing of my circuit is needed but suspect R3's value needs tweaking to suit one's sig gen OP Z.

I've also finished building the wood box/chassis, a nightmare to do but doesn't look too bad.

Andy.
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Old 15th Jun 2019, 7:15 am   #29
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Default Re: Valve preamp.

Hi Andy, I have built and use a reverse RIAA filter for a quick check to see if I am in the right ball park. Once I am happy I then run a full response plot.
My filter has a 100:1 attenuation at 1kHz which I factor in.
I will try and look out the schematic and a photo later (just off to the car boot)

Peter
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Old 15th Jun 2019, 11:43 am   #30
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Default Re: Valve preamp.

Back from a successful trip to the car boot with a couple of radios (I will post about them separately)
Attached is the details of the Reverse RIAA filter I use and a couple of photos. Also attached is the actual frequency response.

Peter
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File Type: pdf reverse riaa filter.pdf (68.1 KB, 21 views)
File Type: pdf Reverse RIAA_20190615_0001.pdf (750.5 KB, 20 views)
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Old 15th Jun 2019, 4:15 pm   #31
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Default Re: Valve preamp.

Thanks Peter. Your RIAA inverse filter uses different value's to mine. I redid mine after finding an 820k R and two 1800p caps, this gave me roughly 830k 820k with a 10k in series and 3n6 in parallel and a 75k and 1n II. I checked the plot roughly and it coincides ish with an inverse curve I found online. When used on the IP of the VW amp I got a flat response, so all's well there at least for a quick test.

I've managed to also get the OP clean by improving my test rig, IE RCA in with very short wire onto V1 g1 and a BNC out. I clocked that my scope test lead had 50hz, about 60mV P-P just sitting disconnected, 20mV more than another one I had. I also turned off the DC heater supply to see how that effected things, no change.

So after sorting out the test setup I properly tested the VW amp and found gain is crap, EG 540mV RMS for 1.5mV RMS in, it clips if IP is raised, I suspect something is off as it should be capable of more than that.

After sorting that out I'll test your design next, Andy.
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Old 16th Jun 2019, 9:56 am   #32
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Default Re: Valve preamp.

A few questions on preamp architecture...
1) Phono amp passive or feedback RIAA filter? I've read passive designs are bad as they have high gain in the first stage which is then chucked away in the RIAA filter.
2) In this amp there will a phono amp and line amp, in the designs so far looked at most have a cathode follower buffer, is it ok for the phono and line amps to share a common KF?
3) Where is the best place for the volume/attenuator to go? After the line stage? Before the KF buffer? Also what value with how much attenuation? So far 100k/60dB are the figures I've arrived at.
4) I have some far out switches that would make good attenuators, is an attenuator better than a standard log pot, if so it it better to have constant Zin or out?

Andy.
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Old 16th Jun 2019, 11:39 am   #33
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Default Re: Valve preamp.

The argument about passive RIAA is to do with headroom, and that has to do with the gain structure. You need to arrive at a situation where the noise is dominated by the input stage, and there is sufficient headroom in the pre-RIAA filter so that clipping is not possible.

If headroom is an issue, then wrapping the RIAA around the input stage is the way to go.

Douglas Self reckons that passive RIAA is a "daft idea", his point being the loss of HF headroom (18dB at 20kHz) in the flat stage before the passive RIAA. He also makes the case that you need two gain stages rather than one, with distortion consequences.

However that does not recognise that there is very little energy above 5kHz, and clipping is no more likely in a passive RIAA as compared with feedback RIAA. And with good design principles the distortion is not likely to be an issue.

The only meaningful point is that the series resistor in the passive RIAA needs to be as low as practical to prevent that contributing to the overall noise. That means that whatever drives the RIAA needs to have sufficient driving capability to manage the relatively low RIAA impedance. Calculation will tell you what is needed there.

Craig
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Old 16th Jun 2019, 11:43 am   #34
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Default Re: Valve preamp.

On point 4, I'm in favour of switched attenuators. The simple reason is that track matching is usually pretty lousy with log pots. Whereas with switched attenuators the matching is determined by fixed resistor tolerance, so L-R gain matching is much much better.
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Old 16th Jun 2019, 7:19 pm   #35
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Default Re: Valve preamp.

The design I posted in post No5 of this thread uses passive equalisation and true enough it has 2 gain stages, one before and one after the equalisation.
I have tested it on the bench and headroom certainly has not been a problem.
This has been used as my main Hi-Fi setup feeding a home designed 10W valve amp for a couple of years now and I am very pleased with it.

Peter
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Old 17th Jun 2019, 6:28 am   #36
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Default Re: Valve preamp.

The Valve Wizard phono amp I tested did clip at 0.5v RMS and what's more clipped when the IP was increased a tad, this might have been down the build quality or something I did wrong though to be fair.

What is the OP of your circuit Peter BTW? I need at least 1.5v RMs to drive my big valve amps to full power OP. A preamp with a OP 6v RMS is what I'm aiming for.

So attenuator = good, but where do I put it? Obviously not after the buffer, need to work this out first so I can finalise what Z to make it and what circuits to use.

Andy.
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Old 17th Jun 2019, 6:56 am   #37
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Default Re: Valve preamp.

Hi Andy, you have got me curious, I will put the pre-amp back on the bench and check the output again.
In my setup the volume control is at the input of the main amp, so after the cathode follower in the pre-amp.

Peter
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Old 17th Jun 2019, 7:38 am   #38
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Default Re: Valve preamp.

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On point 4, I'm in favour of switched attenuators. The simple reason is that track matching is usually pretty lousy with log pots. Whereas with switched attenuators the matching is determined by fixed resistor tolerance, so L-R gain matching is much much better.
Another advantage of switched attenuators is that if you want to use a feedback type volume control circuit, you can distort the law to suit the circuit and get a nice linear change in dB versus rotation.

Off the shelf log pots are really 2 linear slopes grafted together so switched resistors allow a smooth progression.

You can also use nice low noise metal film resistors.

As to constant Zin and Zout, don't bother. The attenuator is buried in an amplifier which can be designed around it. A switched tap from a string of resistors will do fine. Put some effort into preventing clicks and pops when the switch is moved - even if you have a supposed make-before-break switch.

David
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Old 17th Jun 2019, 9:53 am   #39
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Default Re: Valve preamp.

These guys https://khozmo.com/index.html do some stunning switched attenuators which start at just over 100. That might sound like a lot, but the construction has ball races on each end of the shaft, and 48 switched positions.

They do a kit for an IR remote control one using relay switching and 64 attenuation steps with a numeric indicator, but that is quite pricey.

Back in the day (1976) I built a preamp when I spent a summer at CERN using their PCB facility to make the boards. I used a switched attenuator, but only 12 way switches were available. So I put another wafer on, and used that to fire a flip flop, that switched in or removed an additional resistor using a relay. So turn to zero and it would switch to the quiet range, turn to zero again and get the loud range. Etcetera. So I synthesized a 24 position attenuator using a 12 position switch.

Craig
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Old 17th Jun 2019, 11:42 am   #40
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Default Re: Valve preamp.

Ball races on a volume control??
How novel.
Its VERY like spikes on the window frame to stop harmoan-icks.
I pay about $25 for beautifully made switched attenuators.

I switch them left and the sound level goes down, I switch them right and the sound goes up.
one hundred quid ( about $170 for me) is almost a weeks pension.
MUST sound superb as the ball races dont spin, or for that matter I doubt would even rotate a single ball 360 degrees.
Craig!! where are you going ?
WITH my best wishes

Joe
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