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Old 11th Mar 2018, 4:31 am   #1
joebog1
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Default Pre-amp for Golden ears

So most of the design phase of the Laboratory Golden Ears amplifier is done!
The chassis is "mostly" made, its been designed ( for better of for worse) and is waiting for weather to improve to go further. I am still "convinced" it will work just fine.

Now I will need a Phono stage to go with it.
Ohh Noo!!!!
says my post reader and approver ( David I think) .
I am looking at using some twin triodes designed specifically for RF, in cascode format. BUT being for RF they are decidedly microphonic which is nowhere near as important, as it is for audio.
My question:
I need to isolate the valve base (s) from the chassis, so that I wont suffer physical "modulation" of the valves. (6BQ7A )
Normally when I make a shock mount valve base, I use two rubber grommets fitted into holes in the chassis ( plate), with brass sleeves inserted into the grommets, a 3mm or 6BA screw and nuts and flat washers, are used so that the valve and its base is "isolated" from the chassis. That is just one idea!!!
I am asking the hundreds of thousands of hours of experience within this forum, for other ideas.

Ohh! I have tried "spring mounting" using small coil springs, and that idea is not feasible, at least as far as my experiments go. I always end up with the microphonics sounding like a reverb spring, even when I damp the springs with either heatshrink or foam inserts into the springs.

For any who are interested, the article I am basing my design on is:
Cascode Preamp Improves Signal-to-Noise Ratio
Audio magazine October 1955
by M.V. Kiebert, Jr

With many thanks in advance,
Joe
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Old 11th Mar 2018, 12:59 pm   #2
Radio Wrangler
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Default Re: Pre-amp for Golden ears

To combat microphony, your 'shock mount' needs to achieve good mechanical isolation across the entire bandwidth it is going to be subjected to. That makes it a big deal because then you have to make sure the primary suspension resonance is below the lowest frequency in the room.

A valve and its holder are quite light so the springing must be very soft.

Alternatively, add mass.

And you don't really need to isolate one preamp pard from each other so why not mount the whole preamp on some sort of suspension? Remote the switches using relays. Motorised pot?

Otherwise, join the dark side and use a few transistors.... they can do cascodes, too.

David
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Old 11th Mar 2018, 1:37 pm   #3
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Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Radio Wrangler View Post
Alternatively, add mass.
I occasionally used to design inertia-block machine suspensions for vibration isolation. The thought of doing this for a bunch of valves made me smile. (Although - I once screwed a rubber-mounted xlr socket to a church lectern for similar reasons. But alas, Fourier tells us that the Dirac delta function contains all frequencies, including those around the LF mass-spring resonance one has just introduced into the system. Bad news for Bible-thumpers ).
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Old 12th Mar 2018, 6:56 am   #4
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Default Re: Pre-amp for Golden ears

Is it necessary to use an RF double triode? The ECC83 has been used in AF cascode circuits. And Rogers used the ECC808 for the cascode input stage of its Master Mk II control unit. I think that the ECC807 was also specified for cascode applications.

Amongst the RF double triodes, I don’t think that low microphonics was a specific property of the 6BQ7. Apparently the 12AT7 did have low microphonics by RF standards; GE designed it for use as an FM and VHF TV mixer-oscillator; for the FM and split-sound TV applications, low microphony was highly desirable, and in this regard it was said to be noticeably better than the 6J6.

Would a cascode input stage – presumably with shunt feedback – be any quieter than say a series-feedback triode pair with DC heating?


Cheers,
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Old 12th Mar 2018, 9:59 am   #5
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Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Synchrodyne View Post
Is it necessary to use an RF double triode? The ECC83 has been used in AF cascode circuits. And Rogers used the ECC808 for the cascode input stage of its Master Mk II control unit. I think that the ECC807 was also specified for cascode applications ...
I agree it would definitely be worth considering a valve which was designed specifically to do this job. I couldn't put it better than Morgan Jones does in his book Valve Amplifiers:

"An important point to note with cascodes is that there is only one valve that is really suitable for use as a cascode; this is the ECC88/6DJ8 or the E88CC/6922 (special quality version) which was designed to be used as a cascode. Try other valves by all means but do not expect the performance to be as good."

As well as considering the relationship between the resonant frequency of your mounted valve assembly and the noise spectrum in the room you should also take into account the resonances in the metal structure of the valve itself. As far as I know the only example of Mullard mentioning this specifically was in the case of their EF86 where they did everything they could to minimise the microphonics. They said http://www.r-type.org/addtext/add036.htm:

"The electrode structure has been made particularly rigid to keep the microphony of the valve to a very low level. There are no appreciable internal resonances below 1000 Hz, the vibration at higher frequencies being effectively damped out by the chassis and the valveholder."

I note that the ECC88 is a frame grid valve which, all other things being equal, is a stiffer structure than the usual 'grid rod' approach.

If you want a softer suspension than rubber grommets then you could consider cutting a relatively large hole in the chassis and fastening a disc of thin rubber sheet over it. The valveholder could then be mounted in the centre of that sheet. One issue with soft suspensions though is the connecting wires to the valveholder pins. There's no point in mounting the holder softly only to short (vibrationally) the whole soft isolator out with stiff wires coupling it to the surrounding circuitry.

When I was a child I remember visiting the house of a dedicated audiophile who was concerned about microphony driven by the loudest source of sound in his listening room - stating the obvious, this was the music he was playing. He simply put his record deck and electronics in the next, much quieter, room and fed the speaker leads through a small hole in the wall. I think he even organised a delayed lowering device for the tonearm so he didn't miss the first few seconds of the music as he was walking back to his listening chair !

Cheers,

GJ
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Old 12th Mar 2018, 2:46 pm   #6
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Default Re: Pre-amp for Golden ears

The 6BQ7A was also designed for use in cascode circuits and can be seen as a predecessor of the ECC88, but only has half the transconductance.
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Old 12th Mar 2018, 3:25 pm   #7
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Default Re: Pre-amp for Golden ears

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Originally Posted by GrimJosef View Post

"The electrode structure has been made particularly rigid to keep the microphony of the valve to a very low level. There are no appreciable internal resonances below 1000 Hz, the vibration at higher frequencies being effectively damped out by the chassis and the valveholder."


GJ
That is probably why EF86's give little trouble in phono preamps. All the ones I have had have had no issues with microphonics, and no soft mountings were used. I also found the 5879 equally good. So it might be an idea to have a valve intrinsically good in this respect and go for rigid chassis mount than a valve with difficult resonances that needs to be hidden away heating itself up in rubber sleeves and soft mounts.
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Old 12th Mar 2018, 4:15 pm   #8
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Default Re: Pre-amp for Golden ears

For what it is worth, the resonant frequency of a spring/mass (which is what an isolation mount for a valve is) is given by (1/2.pi).root(g/B)

Where g is acceleration due to gravity (9.81m/s^2) and B is how much stretch there is in the isolation springs. Therefore the resonant frequency depends only on the stretch of the suspension springs.

So if the springs stretch by 1 cm (10^-2 m), the resonant frequency is 5Hz, which looks fine. If they stretch by 1mm the resonant frequency is 16Hz, which is probably still OK.

Mullard did quite a lot of work on internal resonances and hence microphony in a valve published back in the 1962. Download from where I found these http://www.thevalvepage.com/valvetek/valvetek.htm

Craig
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Old 12th Mar 2018, 11:11 pm   #9
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Default Re: Pre-amp for Golden ears

So..... what about a pair of triode connected EF86s in cascode, then? Isolated (dc?) heater supplies could take care of any Vh-k issues.
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Old 13th Mar 2018, 12:20 am   #10
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Default Re: Pre-amp for Golden ears

You don't have to use triodes to make a cascode...

David
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Old 13th Mar 2018, 1:21 am   #11
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Default Re: Pre-amp for Golden ears

Quote:
Originally Posted by Craig Sawyers View Post
For what it is worth, the resonant frequency of a spring/mass (which is what an isolation mount for a valve is) is given by (1/2.pi).root(g/B)

Where g is acceleration due to gravity (9.81m/s^2) and B is how much stretch there is in the isolation springs. Therefore the resonant frequency depends only on the stretch of the suspension springs.

So if the springs stretch by 1 cm (10^-2 m), the resonant frequency is 5Hz, which looks fine. If they stretch by 1mm the resonant frequency is 16Hz, which is probably still OK.
It's a bit of a misleading formula, this - the resonant frequency depends on the spring constant and the mass. Nothing to do with gravity, the resonant frequency will still be the same on the moon. But! The Spring constant is also the extension divided by the force to produce said extension, hence the g/B calculation also works.
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Old 13th Mar 2018, 1:54 am   #12
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Default Re: Pre-amp for Golden ears

Thanks for all the answers and ideas!!

I quite agree with all the comments abouit using ECC83/12AX7. Has anybody had a look at the pricing lately?? Im old and slow NOT stupid, and paying $100 yank bucks for a "supposed" NOS Mullard or AWV version has two problems!!
1. I need at least 10 of them.
2. I need at least a few replacements for failures.

I have tried the russian and chinese stuff and it makes more sense to me to use noise diodes instead, GAIN taken into account!!!.

I posted the link to the origional article using the 6BQ7A and I pay $2 for them Brand new in origional packaging, made in USA by Sylvania, and I have access to thousands of them.
6922/6DJ8 has similar problems to 12AX7/ECC83. I do have a SMALL stock of 6DJ8 and 12AX7, but I dont have sufficient quantity to remove the noisy ones, and that means I cant cover any failures down the track. As far as pentodes go, I am listening to EF86's at present and agreed they are not microphonic, but they do have hum problems. Admittedly they are probably my fault, as I made the whole amplifier integrated. I ALWAYS use DC heating in ANY low level circuitry!!. With the Golden ears amp project ( Im thinking my last amplifier for myself) I have done the following:
1. A separate power supply chassis.
2. Very underrated toroidal power transfomers with both pri/sec electrostatic screening, and an external flux band.
3. Large and very low ESR electrolytic capacitors.
4. Large and well screened filter chokes within the power supply.

1. Power amplifier will be at least 750 mm away from the power supply.
2. Additional filter capacitors to buffer the umbilical feed, and an additional choke for the minor HT levels within the amp, with associated big capacitor.
3. As logical as I could ( think ) make the layout.
4. All low level valves will be in screened cans.
5. DC heating for all but the 5B/254's.

Preamp design is still in progress, but will be the minimum required to work and drive the Golden Ears power amp.
As an addition, I want to add a small triode power stage to drive the
"deaf as posts" digital garbage sound cards that are about, some requiring at least 5 volts of signal at about 2K input impedance. I have wound transformers specifically to this end.
I have many LP's that havent, and probably never will be released onto a storage medium like a seedie, and I constantly get requests to copy an LP onto digital media.

I dont use "tone controls" or loudness compensators on any of my amplifiers, as the superb Tannoy speakers I use do not require any. Even listening to early Beatles releases, which could be classed as "screetching treble" recordings.

Back to pre amps!! ( Sorry mods)

Some of you may have already worked out that I am willing to try new ideas!! Following along on this idea, I have spent about 3 years searching out any and all the information I can on this final designor plagiarised build. As far as how far back I go, is limited by RIAA specs. i.e. I dont play 78's although I have many hundreds.
I can design my own filters, but fitting switches to correct for the umteen dozen different recording characteristics is not worth it in my mind, considering of the 78's I do have, not many contain any "music" I am interested in.
Joseph Marshall also had cascode designs for preamps in the early 1950's, all in the search for lower noise. Hence my thought to try 6BQ7a's. It was already recognised back then, that some valves were more suitable than others for this class of service. Triode connected EF86's only deliver a low noise half of a 12AU7, in other words pretty low gain. Something in the vacinity of 20 rather than 60 or so for more suitable types which dont need extra amplification along the way.

I also have a large stash of 6BR7 which is Brimar's answer to GEC's EF86. In any case the 6BR7 is a much newer design than an EF86, albeit using similar design and construction techniques. I have not tried them in triode mode however. Twin triodes have both required elements in one bottle which simplifies heater wiring and reduces current required as an extra bonus.

GJ has an interesting idea re: a "rubber sheet" mounting idea!!. Its why I posted the question in the first place. Davids idea re: using some sand blobs is out!! Sorry David, the only concession I have made to sand in the whole power amp design is to use fast recovery diodes in the rectification circuit. Again because decent rectifiers have been priced out of this world. This new preamp has vacuum rectification though, as I have large stocks of Bendix Aircraft Company JG-6203 aka CV5009 rectifiers, again because I get them cheap, and there are hundreds available.

Thanks again to all for all the ideas and suggestions, and remember!! I am an aussie and I am very hard to offend .

Best regards
Joe
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Old 13th Mar 2018, 9:39 am   #13
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Default Re: Pre-amp for Golden ears

It's also worth remembering that the isolation may have to be applied in all three dimensions. The main electrodes in many valves are vertically oriented so the electric fields are horizontal. It may well be that horizontal vibrations play more of a role in changing the electrode spacings, and therefore affacting the electronic signals, than vertical ones.

Cheers,

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Old 13th Mar 2018, 11:44 pm   #14
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Default Re: Pre-amp for Golden ears

Thanks again GJ,
I already have it in mind to orient the valves horizontally, as I already have a nice 2RU case that is laying around and gathering dust. Its "too short" to stand the glass up, so I have already planned laying them down. This also coincides with the cooling slots, which are only 50mm wide at the front and back of the enclosure.

Joe
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Old 14th Mar 2018, 1:00 pm   #15
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Default Re: Pre-amp for Golden ears

Hi Joe, did you say you have a link to the original article on the internet? Can you share as I would be interested in reading it. I built a simple 2 stage triode (ECC83) phono input and the noise is pretty low.
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Old 14th Mar 2018, 1:16 pm   #16
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Default Re: Pre-amp for Golden ears

Does this answer your question?

http://www.americanradiohistory.com/...o-1955-Oct.pdf

He tells you in the #1 post where it can be found, I thought everyone knew about The American Broadcast History website. If not then be prepared to get lost for days, weeks or years. Loads of magazines there including Wireless World, ETI, Practical Electronics, Radio Constructor, Tape Recorders and tape recording, that is just a small sample.

The Audio Engineering magazine this came from has lots of circuits it moves away from DIY in the early/mid 1960s.
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Old 14th Mar 2018, 6:04 pm   #17
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Thanks, Now you have ruined my life... the site is new to me.
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Old 14th Mar 2018, 7:52 pm   #18
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Quote:
Originally Posted by PJL View Post
Thanks, Now you have ruined my life... the site is new to me.
Only too glad to help!!!!

I do not see why I should suffer alone..........have fun!!!
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Old 14th Mar 2018, 9:33 pm   #19
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Default Re: Pre-amp for Golden ears

Hey Joe have you ever tried tube damper rings, I haven't! are they of any use, or would they help a little?
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Old 14th Mar 2018, 11:14 pm   #20
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Default Re: Pre-amp for Golden ears

Yes tritone I have used damper rings. Read the advertising!! Ones costing as much as 50 quid work much more gooderer than 20 quid ones. A fellow that brought me an amp for repair had them stuck all over the glassware in the amp. After careful examination I discovered that ordinary hydraulic "O" rings even triple the output power of your 1960 Vauxhall, if you fit them to the car radio!!! They prevent punctures as a sideline as well. Well you dont want punctures in your whitewalls, do you?.

The bloke was convinced they worked, I couldnt tell any difference.

PJL, I did post the article link, as has been pointed out. I am quite fascinated by Joseph Marshall's designs, as he goes to great lengths to get as close to perfect as was possible in the early to mid 1950's. Components have improved probably 1000 times since then, yet he achieved superb results with carbon resistors and paper capacitors. He did use Acrosound transformers though in his output stages, so even in todays terms they are very hard to beat. Being a bit of a perfectionist myself, I have decided to build myself one last amplifier, and have decided on his last ( that I could find ) design, namely the Laboratory Golden Ears. I have a quite considerable collection of articles from all over the world on amplifiers and preamps, but the knowledge on this forum is surely much greater than magazine articles.
I started with winding myself a set of transformers with the last of, or nearly so, the best bits and pieces I had laying around.
I did design and wind transformers of all types for 40 years or more, so I made the best effort I was capable of, and ended up with two 250 watt stacks of iron ( 5% Si GO ) so running them at 50 watts should prove quite nice. They are 9 section 5 Pri and 4 sec with staggered interconnects to balance out the DC resistance as far as possible, without going to the lengths of partially winding the transformer with resistance wire to achieve "perfectly balanced resistance".
Back to the forum:
I asked the question to get as many opinions, and as much advice as possible. There is no such thing as too much information, especially when it comes from experience. Also, you never know that somebody hasnt already done what I am attempting to do.
I have bogged up the chassis, and am just waiting for the weather to settle down to paint the chassis, and as its only a couple of days building to complete the amp and power supply, I will need a preamp to see how it works. Driving a superb amplifier with an oscillator doesnt sound too inviting for long term listening. I do have some sand preamps here that I can use, but I dont like the idea. The present amplifier I use, has a built in phono stage using pentodes for the mag input stage, and it has hum problems. My own fault as I previously stated by building it into a single chassis with the power stage, and the power supply, all on a steel chassis. The power amp itself is extremely quiet, but the phono stage sucks, even though its inside a steel screen under the chassis, has DC heaters, and valve covers in an attempt to shut the hum down. I used 6BR7 pentodes and selected the quietest ones I had in my stash of about 25. SSooo that led me to more reading and study to find
"The Ultimate" audio system. Im 65 now, half deaf, have very poor frequency response, but can still discern good sound from bad sound. And in any case my kids will inherit the amp and a few thousand LP's along with the rest of my audio clobber.
Just in passing, I use a set of Tannoy Cheviot speakers, and an interesting article in Hi-Fi World November 2017 is good reading. Tannoy have re released them and you can purchase a set for the paltry sum of 5,199.98 pounds sterling. ( Whats the 98 pee for?, the sticky tape on the packing cases? ) I bought mine for 100 bucks aussie, but I did have to fit new surrounds, as is the case with Tannoys after a few years in Australia.

I will keep you up on developments,
Joe
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