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Old 8th May 2022, 11:14 pm   #1
nzoomed
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Default Thoughts on this HiFi line amplifier?

Looking at building this circuit here. Looks fairly simple and has a bypassable tone stack.
http://www.dogstar.dantimax.dk/tubes...s/toneampm.gif

More info on the whole unit it comes from:
http://www.dogstar.dantimax.dk/tubestuf/tonecont.htm

I will probably be using my own RIAA phono stage instead of his which uses E88CC/6922/6DJ8 tubes.

I was looking at adding a balance control to this which I would insert at the end, any recommendations on where the best placement would be would be helpful.

Also a bit stuck around VR103 in the schematic, this looks like he is using 3 specific neon lamps that were designed to work as a cold cathode voltage regulator, from what ive been reading they run around 75V or so.

Would this mean that its regulating 225V at the anode of this?
Im wondering if there is any voltage regulator in the OA family or similar series that could be suitable?

I dont need any of the FX loop or tape monitors, etc, so I will remove that to simplify things.
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Old 8th May 2022, 11:33 pm   #2
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Default Re: Thoughts on this HiFi line amplifier?

an OA2 and an OA3 in series will give you 225 volts. They will use a lot more power than three NE2's.
They also require valve bases.
NE2's are extremely common and cost a few cents each. Jaycar has them, for one supplier. there are many more.

Joe
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Old 9th May 2022, 12:19 am   #3
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Default Re: Thoughts on this HiFi line amplifier?

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Originally Posted by joebog1 View Post
an OA2 and an OA3 in series will give you 225 volts. They will use a lot more power than three NE2's.
They also require valve bases.
NE2's are extremely common and cost a few cents each. Jaycar has them, for one supplier. there are many more.

Joe
So they are just a common off the shelf neon lamp? I thought they may have been one of a particular spec because i was reading how they are not easily to get anymore or something.
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Old 9th May 2022, 1:51 am   #4
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Default Re: Thoughts on this HiFi line amplifier?

Used as a voltage offset in a feedback path to allow for the difference in operating voltage of an anode and a grid.

As it is, I expect that circuit will make some rare pops and bangs when switches are operated.

David
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Old 9th May 2022, 3:20 am   #5
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Default Re: Thoughts on this HiFi line amplifier?

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Originally Posted by Radio Wrangler View Post
Used as a voltage offset in a feedback path to allow for the difference in operating voltage of an anode and a grid.

As it is, I expect that circuit will make some rare pops and bangs when switches are operated.

David
I did wonder about the possibility of that happening, but I was sure i read somewhere there that if the knobs are centred, you wont notice much difference in volume when switched on or off.

Not sure if ACG needs to be connected to as marked here. The phono stage has a point marked this that im assuming it connects to for some reason.
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Old 9th May 2022, 5:48 am   #6
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Default Re: Thoughts on this HiFi line amplifier?

NE2 neon "bulbs".

Initially designed as pilot lamps, they soon became very desirable in early computers amd counting circuits. They also last a very long time if used within their ratings.

The photo shows a small part of my "neon " collection. If you cant find any PM me and I will send you a few.

A small problem with NE2, is that they dont always strike ( light up ) at the same voltage.
For the circuit you post it doesnt really matter, but there were many same little tiny bulb neons that have very specific ratings. I have boxes of MIL spec ones that are different voltages to what you require. That said, the exact voltages are not so important.
What is important is that once "struck" the voltage will be held within very strict limits so that gain and currents through you valves doesnt change. That gurantees your volume doesnt jump up and down, and noise is fairly well controlled too.


Joe
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Old 9th May 2022, 6:34 am   #7
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Default Re: Thoughts on this HiFi line amplifier?

To help many smll neons ionise and get going, there were often some radioactively frolicksome additives to their gas content. Over time, half-lives go by and their effect reduces, leaving increasing strike voltages. HP and Tek used these bulbs as voltage references in power supplies in their instruments. They are now troublesome and NOS bulbs are no better.

Also, the current flow in a gas discharge tube has long been used as a noise generator. Words you don't expect to find in a pramplifier. Care is needed over filtering BUT with much capacitance around, it's easy to kick of a relaxation oscillator. Use with caution!

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Old 9th May 2022, 7:54 am   #8
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Default Re: Thoughts on this HiFi line amplifier?

Quote:
Originally Posted by joebog1 View Post
NE2 neon "bulbs".

Initially designed as pilot lamps, they soon became very desirable in early computers amd counting circuits. They also last a very long time if used within their ratings.

The photo shows a small part of my "neon " collection. If you cant find any PM me and I will send you a few.

A small problem with NE2, is that they dont always strike ( light up ) at the same voltage.
For the circuit you post it doesnt really matter, but there were many same little tiny bulb neons that have very specific ratings. I have boxes of MIL spec ones that are different voltages to what you require. That said, the exact voltages are not so important.
What is important is that once "struck" the voltage will be held within very strict limits so that gain and currents through you valves doesnt change. That gurantees your volume doesnt jump up and down, and noise is fairly well controlled too.


Joe
Theres tons of those at our jaycar if thats all I need.

I was wondering what else would be tidier to use. Maybe there is a suitable zener diode, or else I would have to string a few of the things together probably anyway.
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Old 9th May 2022, 10:26 pm   #9
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Default Re: Thoughts on this HiFi line amplifier?

Zener diodes would be much better. BUT they can still be noisy if you used the wrong voltages to add up to what you require. Zener diodes are a whole world of their own, having different characteristics with different voltages. They can supply or better worded. regulate over a wider current range than a neon pilot lamp. There are many good books on zener diodes if you really want to learn about them. Be warned, you will find 10,000 books on them, most of which have the same information.

My advice ( only mine ) is to follow the circuit as is, and see how it works. When you buy NE2's buy a dozen, or even 25 and test them to get as many "similar" ones together to use for your project.


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Old 9th May 2022, 10:51 pm   #10
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Default Re: Thoughts on this HiFi line amplifier?

Zener diodes can be noisy, but they don't have a negative resistance region so the noise can be sunk by a largeish bypass capacitor without inadvertently creating a catastrophic relaxation oscillator. Another problem with Zeners is their temperature characteristic which can mean the voltage varies more than we might want. The value of the characteristic depends on the device voltage, changing sign somewhere around 5-6V and (obviously) going through zero in the process. But we'd need to stack a lot of, say, 5V6 devices in series to get up to the sorts of voltages this circuit calls for !

Cheers,

GJ
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Old 10th May 2022, 12:35 am   #11
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Default Re: Thoughts on this HiFi line amplifier?

In that circuit, the neons feed into a fairly slow 22k/10uF lowpass filter which removes most of the noise in the audio band, so this feedback loop is purely for DC operating point stabilisation. However Zout of the signal stages is not terribly low, so some neon noise adds in at the output coming bakwards out of the neon string.

AC feedback comes by a different route, and is AC coupled so it doesn't disturb the DC conditions.

Together this is not bad, but there is a low frequency noise generator on the go. Hopefully its cutoff is so low that the power amp doen't respond to it.

This preamp employs shunt feedback, which has an inherent noise disadvantage compared to series-feedback circuits (The John Linsley Hood versus Hugh Walker letters battle in wireless world). This effect really comes into play with MM pickup stages (this has one not shown on the diagram.)

Missing are leak down resistors so that when the tone in/out switch is operated, no DC differences generate transients. Similarly the input and tape monitor switches.

The tape monitor switching is odd. Use the switch and it changes which signal is sent to the record input of the tape machines. It can do tape-to-tape dubbing but it can't handle 3-head tape machines and allow you to monitor source or off-tape. You'll get strong tape echo effects.

There are enough bits wrong with this design that I would neither build one, nor would I use it as a starting point for developing something with its foibles fixed. Easier to do a clean paper design.

It is principally aimed at those who MUST use valves as an absolute imperative, and at those who think the fewer valves the simpler and better. I'm sorry to say this, but it's a design for a boutique product where performance has been compromised in order to tick certain boxes on a fashion checklist. It's not terrible, but it's just disappointing that it wasn't done better. If I'm going to spend a few days of my life-expectancy building something, this isn't it.

David
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Old 10th May 2022, 1:14 am   #12
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Default Re: Thoughts on this HiFi line amplifier?

As GJ correctly points out, and what I was alluding to in my previous answer is to read the book ( s ) on Zener diodes.
That aside, I stick with my idea of using NE2's to get it running and evaluate if the circuit doees what you want.
IF, it works OK you can then discuss modification or improvements to modify it to your taste, or your specific requirements.
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Old 10th May 2022, 1:22 am   #13
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Default Re: Thoughts on this HiFi line amplifier?

Sorry everybody!!! My internet sucks today and posts got delayed and mixed up.

Thanks to David for the explanation.
For nzoomed, David is 500 times the engineer I ever dreamt to be, let alone achieved.

Joe
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Old 10th May 2022, 5:35 am   #14
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Default Re: Thoughts on this HiFi line amplifier?

I tried that circuit, it's a pig to setup IE get the bias of the 12AT7 right, I discounted it and moved on. Having said that Fred Nachbaur knew his stuff, so up to you. Also after trying several times to get line stages with valves working properly I gave up and use a 5532 opamp tone stage in front of a valve amp or valve line stage, something like John Broskie's Constant current draw linestage, simple, easy to build.

There's a few tone controls using valves on the tinternet and they're all a right %^&* to get get working for one simple reason, valves run at high voltage low current, variable tone controls use pots, pots don't like high voltage on em so you have to use a lot of de-coupling caps, these introduce phase changes, phase changes introduce (potential) instability. So if you are not too experienced it can be "fun" getting the circuit's to work. I use the word fun ironically, my actual experience involves a lot more four letter words.

Think on, Andy.

PS, just to dishearten you further ECC81's/12AT7's like to oscillate, so throw one into a circuit with lot's of phase shifts, expect more "fun"

Here's what I built - https://sound-au.com/project97.htm it can be configured for unity gain so you can put a valve linestage in front, Rod sells PCB's, I made my own, more hours of "fun". He lives in your neck of the woods, so no long shipping from the EU or USA. I bolted that onto this - https://www.tubecad.com/2009/03/blog0161.htm he also sells PCB's ,though it's easy to build a point to point linestage. Here's some pics.
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Old 10th May 2022, 6:09 am   #15
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Default Re: Thoughts on this HiFi line amplifier?

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To help many smll neons ionise and get going, there were often some radioactively frolicksome additives to their gas content.
David
Back in the day when I was involved with high power YAG lasers, there was a similar radioactively doped trigger called a Krytron that operated the Q-switch and had to operate at nanosecond switching time. It even had a radioactive symbol on it.

These came from the States, and were subject to all sorts of import controls - and at the end of life had to be returned. Reason was that the main use was as the trigger for things that went seriously bang.

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Old 10th May 2022, 8:21 am   #16
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Default Re: Thoughts on this HiFi line amplifier?

I'm not firing on anything like as many cylinders as Joe implies just at the moment. I've just tested positive for that thing on the go at the moment. Big headache and heavy cold etc. Yuk!

Anyway, the 12AT7/ECC81 was designed as an RF valve with useful gain to much higher frequencies then the ECC82 and ECC83.

Read this as meaning that it can get up to mischief at much higher frequencies. As a consequence the circuit design techniques and construction techniques have to move up to a higher league. The fashionistas and tube-rollers looking for the glamour of more exotic devices all seem to be bothered with audio and haven't really built up the necessary experience in RF design needed to keep RF devices tame. No matter what frequency the intended operating frequency range is, if it oscillates outside this range, then it will still affect things in the wanted range.

Designing a feedback loop is a sort of race. The aim is to get things to behave gracefully at higher frequencies, for the gain around the loop to be rolled off before the phase shift builds up to a point which either causes oscillation or bad overshoots. Usually this trouble is at ultrasonic frequencies. No-one wants this. Audiophools think there is no limitation on the discrimination of their hearing so oscillation at any frequency is offensive. Mathematical engineers do believe their hearing has limits, but understand matters like RF intermodulation distortion, spurious oscillations consuming dynamic range etc etc.

The snag is that the business of analysing or designing loop stability is fairly mathematical and uses concepts somewhat beyond A-level in our educational system. To compound this, it is taught maths first, and only then do they tell the students which didn't run away how useful and necessary it is... and show them the short-cuts!

This knowledge is needed to not only design a good loop, but also to assess one someone else has published. This is highly ironic. The only people who can look at a schematic and say whether it's worth building are those people who could design a good one themselves. Nature can seem very cruel.

Everyone else winds up building things to find out. This can waste a lot of their time and money, but worse, it damages their confidence and that is precious.

Another irony is that the maths to analyse/design feedback loops isn't actually hard. It involves some very weird concepts which scare people off. Some folk have to do it because they need it. I think I may have been too thick to have been scared off.

Anyway, computers allow us to plot charts which allow us to easily visualise these things, adn it comes down to playing chess, moving gain factors, roll-up and roll down frequency breakpoints around to bend curves so that the gain is rolled away before the phase can circle and bite. (very bad pun, no apologies, I've been wanting to use that one for some time).

There are several people around this forum who are comfortable with these concepts, and occasionally post some clues.

Changing the subject, somewhat. Valves look good. Shiny, warm and with a glow like a welcoming log fire in winter (Christmas decorations and Bing Crosby are optional) They are usually mounted up top on full display so everyone can appreciate their charm.

On the other hand, transistors got ahead of them many years ago, especially for the sorts of jobs found in preamps. In power amps, valves have some useful characteristics, especially if overdriven, which transistors don't do so well. But in preamps this isn't going to do anything. So, if you see a preamp or a CD player with a token valve stuck out the top of it, have a good laugh. It's a handy indication of a piece of crap set as bait for the wallets of the gullible. It's like the sort of guy who swears his car is faster now he's stuck go-faster stripes on it. I don't suppose we should laugh, it's a bit sad really.

So, if you want a good preamp, you can do a better job with semiconductors. Over 40 years ago the NE5534 opamp came out. It was spectacularly good for audio, reliable and easy to use. There have been newer parts introduced, with advantages in some specs, but usually at the expense of others. Also the '5534 is cheap. (NE5532 is the dual version) So do a preamp around '5534s. Chances are every record/CD you have that was done in the last 40 years has had its signal passed through lots of these parts. If 5534s are not good enough for you, you'll have to throw all those records away

Decca pressed some gorgeous recordings back in the 50s and 60s, so the 5534 class opamps have no monopoly on good audio, but there is nothing wrong with the 5534 either - it just makes life a lot easier.

My amplifier setup was designed and built 40 years ago. It uses NE5534s in the preamp. I've seen no need to change it in the intervening period, I've just got on with listening to music.

David
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Old 10th May 2022, 10:26 pm   #17
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Default Re: Thoughts on this HiFi line amplifier?

Thanks for the feedback on this.
I know what you mean about 12AT7/ECC81 liking to oscillate, ive heard a bit about this before.
I might look for another design elsewhere I can use instead, I quite liked the idea of a by-passable tone stack, hence why I was interested in it.
I see he has a revised version of this schematic here:
http://www.dogstar.dantimax.dk/tubestuf/instpre.htm
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Old 11th May 2022, 12:47 am   #18
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Default Re: Thoughts on this HiFi line amplifier?

Ah-ha!

It's changed from shunt to series feedback!

There's a resistor in series with those neons so they don't drive as much noise into the output!

The doesn't do what it says on the can tape loop has gone (and taken the input switch with it)

So he's experienced and had to fix most of the things to be found in the first version, or he's had some guidance.

Anyway, it's always nice to see that problems can be spotted ahead of them being discovered by those taking the long way round.

Asimov's positronics never turned up, so it's a shame we don't have complementary valves, they'd make things easier.

David
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Old 11th May 2022, 12:59 am   #19
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Default Re: Thoughts on this HiFi line amplifier?

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Ah-ha!

It's changed from shunt to series feedback!

There's a resistor in series with those neons so they don't drive as much noise into the output!

The doesn't do what it says on the can tape loop has gone (and taken the input switch with it)

So he's experienced and had to fix most of the things to be found in the first version, or he's had some guidance.

Anyway, it's always nice to see that problems can be spotted ahead of them being discovered by those taking the long way round.

Asimov's positronics never turned up, so it's a shame we don't have complementary valves, they'd make things easier.

David
Going by his notes, he seems to at least have some experience.
I think also that he has published this revision as a standalone unit over his other integrated studio amplifier which contained a phono amp and had the tape monitor added, etc.

If this revision is workable I may still go ahead with it, but I would be open to adding any other tweaks or improvements as seen fit to do so.
If i can shift away to another more suitable tube from the 12AT7, I may do that too if not too many mods are required.
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Old 11th May 2022, 5:06 am   #20
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Default Re: Thoughts on this HiFi line amplifier?

Try the 12AT7, just be careful with your layout, it may not oscillate. There's not many dual triodes with a u of 70 apart from the 6SL7 but that's octal. There's a few valves with dissimilar triodes in which would be great for a preamp but non I no of with a gain of 100/70, most are low gain high current triodes for use in TV's, so try the 12AT7, you might have to up the grid resistor value a bit.

BTW, the tone controlI linked to is by-passable.

Andy.
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