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Old 10th Dec 2019, 6:41 pm   #81
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Default Re: Valve preamp.

Hi Andy,
Looks very good indeed, what does it sound like?

Peter
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Old 10th Dec 2019, 9:57 pm   #82
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Default Re: Valve preamp.

It makes me wonder with a lot of the audio work, pre-amps etc worrying about the equalization being correct, when the obvious seems neglected. This refers to some circuits with the outputs taken from 100k load resistors.

For example if you have a stand alone small signal apparatus, it has to be connected to the outside world and send its signal to another apparatus where all the power activities take place.

So it really surprises me that audio preamps get built with output impedances in the order of 100k, since it only takes 150pF of cable capacitance to knock it 3dB down at 10kHz, or 300pF to around 5kHz. I guess its case could be helped if RF rather than audio coax was used and kept short as possible.

So when I look at any circuit designed to export signals down a cable (which could be some quality and some length, a few meters in some cases) and some unknown capacitance, I always look to see if there is a good buffer with a low Z out driving the output connectors, or not. In other words "where is the cable driver ?"

I've also always thought that electric guitars should have a buffer amp in them as a "cable driver", the capacitance of the cable becomes very significant over a few meters and causes significant treble cut with the high Z output from the pickups & controls.

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Old 10th Dec 2019, 11:14 pm   #83
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Default Re: Valve preamp.

That is why I always use a cathode follower output stage!! something in the order of 4 or 5k ohms. I can drive a few feet of cable with negligible hi frequency cut. I have been playing with some of the dissimilar twin triodes to achieve exactly this.
6DE7, 6CS7, a couple of examples. Originally designed for vertical oscillator and driver in one place in B/W TV's. I use the small triode as the buffer/driver for the larger triode which is wired as a cathode follower. 6CS7 is similar to a 12AU7 driver with the big triode approximates half a 12BH7. 6DE7 is close to a 12AT7 with a 12B4A in the same bottle.

Looking good Andy!!

Joe
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Old 10th Dec 2019, 11:47 pm   #84
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Default Re: Valve preamp.

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That is why I always use a cathode follower output stage!! ...4 or 5k

Joe
Good move to get the source resistance for audio work, driving cables, to that sort of value with valve gear.
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Old 11th Dec 2019, 12:46 am   #85
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Default Re: Valve preamp.

A typically neat and well laid out job Andy.

I have to doff my cap to you.

I haven't done anything quite as complicated as that, I find a simpler circuit does pretty much all of it.

Yeah Joe,I'm still fiddling with my transformer output preamp. I'm not really happy with it theres been issues with hum coupling even with attention made to earthing runs etc, so I think it will need ripping out and a fresh start.
I have a simple cathode follower buffer pre which is a CF with an active load, I used John Broskies circuits for this if you know his stuff. And I used one of his PCB based kits for the regulated HT supply which in my view was the best improvement I made to any of my front ends.

I'm unfortunately (or not?) also a fan of decent solid state preamplification and have to be brutally honest and say I have yet to come across a Valve RIAA stage that equals the performance of a well designed and executed silicon job. And likewise it's really difficult to improve on what can be achieved with a couple or 4 op amps in a line stage, right down to easily switchable gain and with maybe a servo feed back circuit which as I understand it, can remove any DC offset and allow a Direct coupled circuit.

But I still fiddle about with valves, I think I just like the romance of them, it's a kind of "Organic" process to my mind.

Andy.
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Old 11th Dec 2019, 1:14 am   #86
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Default Re: Valve preamp.

I agree Andy!! BUT I distinctly dislike the sterile sound of 4000 transistors completely removing the distortion that is actually contained in the recording. I don't own a single music CD !!! I guess I could make an ENIAC out of triodes to equal your 8 pin spider, but there's no fun in that. . The KISS principle can only go so far.
Final laser treatment to my eye this Friday, and ( NON ) wet season is not about to start, ( STILL absolutely zero rain here!!! ) so maybe I will spend some time finishing my pre. Plagiarised of course !!
AS far as transformer coupled output stages go, a valve is more than capable of driving a decent transformer in its cathode leg!!! MUCH less sensitive to noise, and more balls as well. Flatter frequency response, although you do need a bit more current to feed it.
You could even make a push/pull cathode drive output stage with the push/pull transformer being fed by say, a 12BH7, or a pair of 12B4A's, with the transformer between the two cathodes, although heater current goes up.
The centre tap of the traffo is then used as the bias point, with either a resistor and capacitor to self bias, or negative supply with hard bias.

If you are plate feeding the transformer, in my experience, I needed two earths, or ground returns. One for the high current output stage, and a separate one for the low level signal stages. These do of course meet at the power supply.

I agree with your statements about t'other Andy's preamp, But its a tad complex!! I do like the cable lacing.

Joe
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Old 11th Dec 2019, 8:13 am   #87
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Default Re: Valve preamp.

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Originally Posted by joebog1 View Post
the sterile sound of 4000 transistors completely removing the distortion that is actually contained in the recording.
Joe
Now that would be a good trick if it could be done!

If an amplifier could be made perfectly linear, the output waveform would simply be a scaled-up perfect replica of the input waveform. Any distortion, harmonics, etc. present in the input signal would faithfully appear in the same proportions to the amplitude of the signal, at the output. The amplifier would create no distortion, but it would faithfully pass along any in the incoming signal. It doesn't matter what sorts of active devices are used.

Musical instruments, voices, and everyday sounds contain plenty of harmonics, it's implicit in making any waveform other than a sinewave. There is a mathematical analysis and proof - the man who did it died in 1830, so it's definitely vintage technology and far pre-dates valves. So the presence of fundamental frequencies and their various combs of harmonics are not distortion products, they are the music.

If an amplifier, radio link, cutter-head or whatever distorts, it will create harmonic frequency components of every component present in the unmutilated original. These will land exactly on the frequencies of the harmonics already there, and will add to or subtract from them depending on phase.

If some group of evil little transistors wanted to remove this distortion, I can't see how they could determine how much of each component was part of the performance, and how much was created in a non-linearity anywhere along the chain. They could never know how much to remove.

The sterile sound of low distortion equipment is simply its failure to add forms of distortion that the listener likes. What is being heard is an accurate reproduction of what is on the record. If this isn't to your taste, fine. You can add whatever non-linearity you like. It's your equipment, your ears and your choice... and your money. It's freedom. But don't lose sight of what you're doing and why, because if you do, you become vulnerable to the honeyed phrases and strange beliefs of people wanting to separate you from large amounts of cash.

Hugo's point about the output impedance of a stage driving a cable to another unit is right on the money. The length of a few feet of screened cable may be utterly insignificant in terms of wavelengths, but its total capacitance can be significant at audio frequencies due to the impedances presented to it. Playing around with different cables of different capacitances really will produce audible differences. This is usually interpreted to mean 'This cable is better than that cable' but what it really means is that whatever is driving and whatever is loading the cable are very unsuited to their task.

David
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Old 11th Dec 2019, 8:20 am   #88
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Default Re: Valve preamp.

Hi Peter, thanks, haven't really had chance to audition it properly as a cap blew up on the first attempt, but it sounds ok and more importantly drove my two big 120w monoblocks.

"outputs taken from 100k load resistors" ? The OP's aren't taken from 100k R's, there's a CF with a low OP Z to as you say drive any cable capacitance. Not sure where yourself and Joe got that idea from.

Thanks tother Andy. It's not really complicated, just looks it, I'll include the circuit again down below to clarify, one of Broskie's. Essentially it's a common cathode gain stage with a gain of about 7 direct coupled to a cathode follower. The PSU is a bit more complicated or busy than it could be due design being dictated by what parts I had, and what cash I didn't have.

Each IP (2 x 6) goes into a 100k preset to ensure IP levels are the same. This took a few tries to get right, the thing that caused issues was mounting all the presets in a way whereby the little screw heads were easily accessable and secure, this took three attempts and a lot of fettling.

From there each IP goes to a selector SW, then into the 150k stereo attenuator, then into V1. there's two OP's, tape and pre.

The PSU on the back there is a 6.3v htr PSU to power the phono stage, it's a basic LM317 with a TIP3055 series pass.

As it is this preamp will power a valve phono amp and with the addition of a small 12 0 12v tfmr power a trannie phono amp. I'm going to revisit this and knock up a load of circuits and test them all. Couldn't give a rats arse if a trannie amp sounds better, it's a bit like saying I'm not going to ride a bike cos it's made in Japan. If it gets you home from the pub who cares. I took into account what you said Andy about splitting the phono stage and also what David and others have said about an opamp RIAA stage.

Lastly I went for a different approach with this amp in that I didn't worry about THD, frequency response etc too much. The F response isn't too good in the lower frequencies and the circuit can be improved apon by using CCS either valve or sand and by applying some PFB, see - https://www.tubecad.com/2009/03/blog0161.htm for a fuller explanation.

Thanks for your interest, Andy.
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Old 11th Dec 2019, 9:12 am   #89
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Default Re: Valve preamp.

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Originally Posted by bikerhifinut View Post

to be brutally honest and say I have yet to come across a Valve RIAA stage that equals the performance of a well designed and executed silicon job. And likewise it's really difficult to improve on what can be achieved with a couple or 4 op amps in a line stage,

Andy.
We have bought a new sideboard and SWMBO insisted that the valve preamp I talked about in post 42 of this thread had to go so I replaced it with a chip design, a chinese copy of the zerozone preamp, I bought just the pcb and populated it myself, and I have to admit (whisper it quietly) that it sounds better.
Gone is the the very low level hum that I was never able to quite get rid of despite a dc supply for the heaters and screening cans for the valves. More important SWMBO is happy and I am still using a valve main amp.

Peter
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Old 11th Dec 2019, 11:57 am   #90
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Default Re: Valve preamp.

Re using a SS RIAA stage with a valve line level pre with a dose of 2nd harmonic sounds about right, best of both worlds.

Listened to and tested it today and found a few issues, I've some 50hz hum when connected to the power amps, on the scope the preamp alone shows around 10mV P-P of 50hz ripple, this at half volume. On the scope it shows as positive spikes rather than a sine.

The attenuator is rubbish, you have to turn it up pretty much all the way to hear music, and there is a disparity in OP levels between Ch 1 & 2, about 200mV RMS. There is also a dead spot spot on one Ch, which at loudish levels thumps as you turn the control. This could be down to how I put them back together, there are two locating pins that lock the rotors onto the shaft, it seems they only go in one way, how I have them sitting at present, one rotor leads the other, also there may be a dry joint. One other aspect makes putting these back together tricky, it looks like each SW is different and the makers drilled the holes for the locating pins separately for each rotor, the holes are not 90 deg to the shaft, and on one has four holes! I used this girt big SW's cos I had them knocking about for years, they're a bit big really. No worries, Andy gave me a nice Alps attenuator which I'll try - thanks mate.

Connected amp to A PC SA, 2nd harmonic is 40dB down from fundamental, 3rd 50dB. THD is 1.5% on a distortion meter. So, room for improvement. I need to recheck ground wiring, ditch the attenuator and am going to try some NFB, just a tad.

So, performance underwhelming, no worries, I love it when it's not right, it means I can tinker with it more : ) Andy.
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Old 12th Dec 2019, 12:43 am   #91
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Default Re: Valve preamp.

If you are keen to use a switched attenuator I have one here which I bought off Ebay aeons ago, uses Vishay Dale 1% metal films and is a nominal 100k job. It's wired as a proper stepped attenuator in that it switches individual potential dividers for every one of its 23 positions rather than a ladder or shunt (Ugh) job. I can't remember offhand which tags are which but a quick check with a meter would sort that out. Gizza PM if you are interested. I'll have a shufti about cos I remember theres a link to it here. www.rssconsultancy.co.uk/article80.pdf
I'd take a huge pinch of sodium chloride when reading some of it though. I'm fairly immune to the hairy sphericals that accompanies some of this stuff. I have always thought that the principal reason some folks think pot "a" sounds better than pot "b" is down to how good the channel matching is as that may cause the stereo image to wander from side to side slightly as a control is rotated. That and the thorny issue of making sure the resistance/impedance matching is in the ball park too.
Anyway Andy you're welcome to try it. I reverted back to the carbon track potentiometer a while back when I decided I really do prefer to have remote control of the Volume if nothing else.
That reminds me, I'm having a spot of bother with an ebay bargain of a IR remote volume control board with onboard motorised pot, I suspect the IR receiver thing isn't aligned well in the amp chassis and the hole for the beam to penetrate is only a couple of mm dia so its off with the cover and fiddle around with the sensor. its an unusual looking one with no obvious lens side so I think that's the issue. Plus its got 10 inches of 3 core on it as the sensor had to be moved to where I could use an existing hole in the front panel of the amp.
If you try that motorised ALps pot Andy, theres a few ways to do it from a simple wired remote, onwards. I know a couple of lads locally who would bodge summat up with an Arduino board that could be wirelessly controlled with IR or wi fi but thats way beyond my skill set.
This ebay device I'm using just now cost me less than a tenner including postage included the pot too! I doubt it's genuine ALPS at that price but it seems to function fine with no mismatch worth worrying about on the 2 channels. At those prices it's not worth trying to fiddle about with homebrews.
A.
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Old 12th Dec 2019, 12:52 am   #92
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Default Re: Valve preamp.

OK another daft question, why mess about with a bunch of 100k preset/trimmers? Most sources these days are standardised at about the same as CD players output.
Mr Leak had a neat trick of using panel mount pots that could be adjusted with a screw driver on the back panel beside the sockets they serve, these were found on the Point1 stereo that I know of. Han dy when I used a point1 stereo in front of my Stereo20.I may have 2 or 3 in the box somewhere if you want em? You can just use any panel mount pot I guess and saw a screwdriver slot in the shaft.
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Old 12th Dec 2019, 1:11 am   #93
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And finally...…………..

I've gone full circle with my KT88 power amp, it started life as a well regarded Kit some 12 years ago and was the single input version with a 50k pot on the input, an integrated version was available which was basically the same thing but with a selector switch. I've been using it as a straight power amp as it suited my purposes that way.
Anyway I decided to rebuild it as a 5 input integrated when a front panel came up on the bay.
So what we have is, I guess, a "passive" front end in as much as theres a 100k pot on the input. Sensitivity is around 450mV for 35 watts at clip, so fine for just about all my sources and drives the Quad ESL 989 without drama. And its as good as it gets, I really don't think an active preamp stage improves things. No surprises there I guess.
Food for thought......……. although my EL34 homebrew was deliberately built to have a less than 1V sensitivity so a bit of gain in a preamp is welcome there.
This is the amp that I've been fiddling with a remote volume control on.

A.
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Old 12th Dec 2019, 7:14 am   #94
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Default Re: Valve preamp.

A lot to read there mate, but some good points. I tried the attenuator to see what all the waffle was about and I had those girt yon SW's sitting under my bed crying "use me!" I've tried them, they're hairy globed shaped, lesson learnt. (Yes it is a word spellchecker)

"OK another daft question, why mess about with a bunch of 100k preset/trimmers?" Now you tell me : ) It's another case probably of me over thinking things and not knowing what I'm doing. My PC puts out 500mV, AFAIK a CD puts out over a volt, maybe more, haven't got one but that's what I read. On this spurious info I put in the presets, which are adjustable with a little screwdriver on the top of the chassis. Thanks for the offer, but they'll do the job for now and if not needed are all set to max.

I built this preamp mainly cause my two big monoblocks have a sensitivity of 1.5v, that and I had those nice tfmrs laying about doing nothing.

Fitted your Alps pot in yesterday, result = a lot better. Will have a look at fettling a remote at some point in the future. Also made a couple of changes and did some more testing, results a lot better. THD now 0.5% @ 1khz/5.4v RMS OP. Noise/ripple 0.004mV RMS.

Thanks for your IP, I'll drop you a PM or email. Andy.
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Old 12th Dec 2019, 6:53 pm   #95
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A lot to read there mate, but some good points. I tried the attenuator to see what all the waffle was about and I had those girt yon SW's sitting under my bed crying "use me!" I've tried them, they're hairy globed shaped, lesson learnt. (Yes it is a word spellchecker)

"OK another daft question, why mess about with a bunch of 100k preset/trimmers?" Now you tell me : ) It's another case probably of me over thinking things and not knowing what I'm doing. My PC puts out 500mV, AFAIK a CD puts out over a volt, maybe more, haven't got one but that's what I read. On this spurious info I put in the presets, which are adjustable with a little screwdriver on the top of the chassis. Thanks for the offer, but they'll do the job for now and if not needed are all set to max.

I built this preamp mainly cause my two big monoblocks have a sensitivity of 1.5v, that and I had those nice tfmrs laying about doing nothing.

Fitted your Alps pot in yesterday, result = a lot better. Will have a look at fettling a remote at some point in the future. Also made a couple of changes and did some more testing, results a lot better. THD now 0.5% @ 1khz/5.4v RMS OP. Noise/ripple 0.004mV RMS.

Thanks for your IP, I'll drop you a PM or email. Andy.
No worries,
I scribbled off a circuit using a centre detent DPDT toggle switch spring loaded thing so defaults to off, that and a handful of silicon diodes to block reverse polarity and a length of flexible wire and bingo, how very 1960's! You can even use non locking push buttons with a bit of thought.
Easier than IR and a useful tripwire for unsuspecting guests.
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Old 13th Dec 2019, 6:37 am   #96
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Default Re: Valve preamp.

Had a good listen to the preamp and power amps yesterday and am quite chuffed with the result. I hooked up a TT through a Garrard MRM 101 and listened to Spirit Potatoland, The Scorpians 1st LP a Charlie Mingus LP and lastly a bit of Steeleye Span, sounds alright. The setup sounds especially good with old roots reggae, Burning Spear - Man in the hills, Niney - Dollar in Teeth and some 80;s stuff - General Saint & Clint Eastwood and Eekamouse, the bass got me up grandad skanking much to the amusement of our Molly.

The setup does sound better when loud though and even better through my test speaker, 15" Celestion guitar woofer in cab, 8" Celestion mid off a keyboard amp and Fane horns through a random Kef X over. I really must make another as this bodge sounds better than proffessionally designed Mission 720's.

Had a quick look at remote circuits Andy but couldn't really fine owt but did find similar circuits to what you mention. No worries, at present it's not a priority but something to look at in the future.

So, onwards and upwards, next project either a Class A tranny amp which appeals, a quad 807 valve amp or a triode PP jobbie. Plenty of options....

Andy.
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Old 13th Dec 2019, 9:17 am   #97
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Default Re: Valve preamp.

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through a Garrard MRM 101
Andy.
Not seen one of those mentioned in a long time. I've only heard of one other person who had one, but as his previous job had been at Garrard designing it...

David
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Old 13th Dec 2019, 11:27 am   #98
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Default Re: Valve preamp.

Used some of these when I couldn't afford anything else for clicks and pops - the pre-amp is OK but nothing to write home about, and the delay line introduces some "valve-like" even-harmonic distortion. Large clicks in vinyl are reduced to fairly discreet thumps, which is useful, but show it more than a couple of clicks per rev, or worse, 78 crackle, and the output sinks to a burbling mess in short order. The click suppression is done by swift broadband muting of the output, so the limiting case is no output at all.
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Old 14th Dec 2019, 8:46 am   #99
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Re MRM 101 - yes David, not very common bits of kit. On very scratched records it can't do much but does a good job on the whole. Only played 33 LP's Ted so never had any issues.

Andy.
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