Yet Another Limiter Amp (yala)

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alexc

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Doing it again    ;D    Yet Another Limiter Amp  !

So I just acquired some nice and affordable  local iron way down here in lower Tas,  Aust  ... old broadcast stuff .. you know the thing  8)

A bunch of good sized interstage traffos, for a conventional vari-gm cell and a pair of big-ass pp output traffos (25W conservatively rated ..  done pro hifi early 60s Australian tech).  Well shielded fore and aft and good to go.

Going to make a stereo pair of tube limiters in a 3RU long form rack, with an outboard psu  2RU long format rack.

BUT the twist is this - the output section will be a 20Wrms power amp driving some speakers  8ohms.

Basically a pair of 175 type limiters, with interstage bonafides, but a (mildly) big ass output amp.  8)

What could go wrong?

...

The PSU will be a combo of basic tube amp power supply bizness, but with a HV regulator module for screens and such  ..  plus another module for negative and utility rails.

Heater will be dc (unreg)  for the sensitive stuff .. for sure .. possibly also for the power finals ...

This psu unit will use 2 cables to connect with the limiter-amp signal unit  ..  7 pin large style ebay aircraft cheapies, done with heavy duty 'vehicle trailer' cable  (also cheap!) It works out pretty well, and 2 of these gives some room to move, psu cabling wise.

The  intention is to have the psu on the lowest part of a rack  ... some distance  ... the b i g m a n  limiter-amp above.

Best to get the big psu traffos way the way away if possible. This one uses a mighty Edcor - a masterful 400mA of 750Vct with 7A of 6V heater, a seperate bias coil of 100Vac and another for 12V utility with 1.5A

A serious psu hv unit for a serious build!  No contest.

But wait ..  there's more ...  a nice german toroid ...  for the gain reduction cell ...  250Vac winding at 0.2A  of HV, a generous  9V at 5A for dc heaters and a winding with 15-0-15 at 1A per rail for utility :)

The PSU is a big part of any proceedings ..  and this one, is kind of old style modular, in that it powers a couple of signal units..

...

Combining both the limiter and the power amp, in one 3ru,  effectively eliminates one psu section  .. which is great for simplification and isolation. 'Lots of power, further  away'.

*it's basically a lathe-amp!*  (footnote to emrr  and his knowledge of presto tech) 

I'll be driving a variety of speakers ....  typical listening levels for me are around  5W rms and 10W peak  in a room 8mx5m or so. But I do like a lot of reserve for extended bass  ... so 20W max power is about right.

So in the end, a stereo tube mega channel  :)    limiter -> eq  -> power amp -> full range speakers

The diy tube equivalent of 'mind, body, soul'  8)

....

The challenge of course, is noise and hum and schmutz of all kind.  I'm confident I can beat my personal best in this area.

I'm using some interesting remote pentodes, 2 per phase loaded in a quite high Z interstage primary. 

I'm going to try pentode mode first ...  I did this previously in an 'rca style' limiter which came out just beautifully, but it took some doing .  THIS time, I have a 'proper' interstage hiZ to load it with ...

....

I haven't decided on the finals  ...  I have a bunch of nos ge 7591a (19w plate) which would be great for this. Easy to drive with my standard 6SN7  .. or even a 6SL7 would do it great.  What a combo !

Some possible additional twists are to use some affordable sweep tubes for finals  ...  or even some 'direct-heated-triode'  types  :)

The intention is to get really classic, classy  and loud  ..  in one unit  :) But quiiet too! (and cheap)  ;D

...

So .. just in the noodling/acquisition stage now ..  it's winter here and time to proof last seasons' builds. I'll post some pics of the thing as I get some more parts. (I won't jinx it with names yet!)
 

alexc

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The other thing is that I will be able to use this build as an opportunity to mess with alt side-chain types.

My usual is the 6AL5 and 0A2 combo as in the 175b with the dc bias to the rectifier tube,  HiZ style - cap coupled to the final's plates.  I see a control voltage range of something like 7 .. 40V as being what's needed.

The alt is a small ss power amp + diodes with a variable dc-offset.  I've been meaning to mess with this for ages!

And the other other alt, is to do some judicious jagging of the finals' bias ...  old school style  ... but not with a trem osc, but with a GR sidechain.

Nothing new or original in the basic techniques, but member 'Heikki' has done some interesting stuff in this area and I want in  :)

This will definately be my last limiter ....  I 'recycled' around 6 of my earliest builds this past year :mad: so I feel one more 'thing de resistance' is justified.

I'm still experimenting with CF gain cells and grounded-grid diff amps  ...  this exercise may well ping me good in those areas too.

Some genuine vintage Australian elements to this exciting build.
 

alexc

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Actually, it is very liberating head-space-wise  to get really tough on shaking down one's builds  ... 'several strikes and you're out'  kind of policy .. focusses the mind on the best of the fleet, not the worst.

Happily I still have too many units, but cut down by 25% ...  the units which were not really robust  ..  in my much upgraded testing environment.

So now it strictly, 'build one, recycle/sell one' especially where it comes to limiters! 

What amazes me is  ..  most of my builds have survived  after all these years  :)  It's getting harder and harder to cull stuff, either to recycle or to raise funds for parts.

But cull I must!

I closed out my 'music room', downsizing and moving the remnants to my lounge where I spend most of my audio time.

The happy consequence is that I'm using the best of it ...  a lot more  ...  and with more focus  (on great sound).

.....

HENCE the need for a new super duper limiter amp  ...  that weighs 17kg+17kg , uses up to 250VA and needs 11 RU + 6RU isolating empty space inbetween  :)

I mean, you can see the logic, right?
 

alexc

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This build hopefully will chase some low thd figures  ...  plenty of nfb around the 8K:8 20W output traffos back to the driver tube ..  prolly a 6SL7, I think ...  I don't expect any trouble driving the p-p  7591a finals (19 w plate).

The basic 175b topology has very high open loop gain which translates to some very impressive (for the type) thd, closed loop.

I'd like to try some next level distortion minimising techniques this time around ...  for instance, I'd like the driver to be a  long tailed  pair 6SL7 with a negative rail and possibly a tube current source.  That should get the best from the middle tube  ... w i t h o u t going solid state  :)

The symmetric nfb loops from the finals' plates to driver's cathodes/grids  then knock down the schmutz to tiny levels.

SO - for the basic output amp, I'm looking for 20Wrms into 8ohms at 0.1% to 0.2% ..  with enough nfb. 

That means 5Wrms average room listening levels should be less than half of that ..  0.025 to 0.05 % thd or so  :)

A worthy goal in a useful tube amp    ....  the awesome gain reduction cell  is just an added bonus!
 

alexc

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And the final twist in this story of excessive excess, some basic eq is required.

If one is going to the trouble of making an ext rack psu, with 2 cables to the 'main unit' .. some 6RU away, then some basic bax stlye EQ is a good thing  :) and not too costly, resource wise.

So there it is - an EQ in a 1RU with a power cable from the limiter-amp  ...  have to work within a power and noise budget to get it useful ...

The whole thing together *may*  justify the trouble ... I sure do think I can better my personal best in a  ...

2 channel preamp+eq+limiter+power amp+speaker    diy build  :)

I think the approx parts outlay to be reasonably modest .. 

Here's a pic of the initial noodlings  ...  with some recycled rack pieces  :)

....

For these pieces, I'm moving to slightly better  ..  front panel printed films ..  a3  printer instead of the 2-piece a4 prints.

Have to move with the times ...  cheapo style!
 

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alexc

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A couple of more pieces arrived  :)    A few more, the same still to come  - more traffos and more GR tubes  :) 
Another couple of hundred out the door and onto the layout table.

The game is warming up nicely.

....

This gain cell can run from 20mA per phase at idle (max current condition) with a grid-cathode voltage of around  -6Vdc. 

gm drops fairly steeply for about 1 decade, resulting in current down to 1mA per tube per phase ....    with voltage extending out to -20Vdc or so. 

Beyond that, another decade of shallower gm drop, way way down to 0.1mA per tube per phase  ... for the silent types  :)

I think the rp of each phase to be around 2K5 at the low-Z condition  and *alot* more when they flip to the low-current condition.

Have yet to calc and cal properly  :)

  ...  with 2 triode-strapped-pentodes (frame grid 2.5W types) per phase  ... I am loading with 100K/10K interstage traffos, quite heavy duty  beasties  :)  As always, the best 'balance' possible is a compulsory must.

Followed up with a 6SL7 I think ..  at this stage. 

It's a very interesting build prospect ...  I hope I can fit it all in, otherwise my octals will have  to become novals!


 

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alexc

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Thanks!

There are few things I've heard, as a guitar person, that are as thrilling as a really good tube limiter  driving some good tube hifi amps and full range speakers.

If the noise is properly controlled ..  and I mean *really* controlled, the limiter 'swell' can be to die for. It's that 'wind at your back' that really can inspire a player to rip some really tricky moves and make some personal history :)

This is my 6th big (surviving) tube limiter build  ...  with another couple fully decommissioned and now are just .. inventory.

...........

I've tuned my  diy rebuild speakers to have a little less high-freq (> 10K) response .....  once one has the 'hum' tamed in an all tube setup, the 'hiss' is the next thing to look at.

A lot of 'pure' tube  compression can accentuate the high-freq 'pressure' on the ears ..  making things a little hard to take ..  like too much suspense in  one's 'Hammer' horror film  ...

-> my old ears prefer less 'air' or maybe it's the everpresent  solfeggio vibrations in my ear/skull/brain/mind complex
->  might be  mosquitoes and/or  wifi/hiFreq comms  ...  but there aren't mosquitoes down here, so prolly tinnitus!
-> I was very much into the (then) local music in Aust. in the 80s  ..  lotsa gigs  all round .. with those damnable evil twin reverbs

....

And sadly, I have to lose the 6SL7 'middle' tube in this build, for a noval ...  space is tight ...

I'll prolly use 6DJ8 if I can ...  possiblke even 2 of. The signal is knocked down pretty hard by the gain cell output transformer ... but the gain cell can have a surprising amount of gain .. sometimes  ..  hopefully running the middle tube reasonably hard will do it ...

I'll know when I get there  ;D
.....

Looks like amperex holland features strongly in this build ....  it must the will of the 'audio elementals'  :)

I spent many years in the areas nearby that great Netherlands city of Amsterdam, and that home of tube innovation Eindhoven ...  I'm a big fan of the lowlands ...

It feels good to bring elements of Holland and Australia together in this build  ...  with real sixties US power and octals  and righteous movement  metering  :) 

Also of note is some nice sino pcbs for power and a super nice deutsche toroid.  Talk about the UN of 'Sound'  8)  (thanx Richard!)

So tube lineup is

    4x6eh7 -> 1x 6dj8 -> 2x 7591a (octal 19W plate)    per channel signal amp  balanced or 'pushpull' topology
    6al5 sidechain dual rectifier per channel  with regulated dc offset voltage, variable 'dc threshold', 'ac threshold'

and  traffo line up is
      Trimax 10K/25K input, Trimax 100K/10K interstage, Trimax 25W 8K/8ohms  per channel signal amp

Time constant network is an 'attack time' pot + single rotary switch for 'release time' with some cap-resistor combos.

Front panel with Level (vu) buffered meter and a (low voltage current sense)  Gain Reduction (gr) meter per channel

And the mandatory  ..  relay bypass for the limiter section,  per channel  ..  for sanity checks ..  which can generally be a good thing.

...

The Trimax traffos add that glam factor, with the punched brass label plate    ... the gain cell types are big, heavy cast jobbies  ...    and the outputs are nice big pressed-can case types  (you call  that a pressed can?) ..

The holland globo-s with the orange letters and perf plate add the respect factor  ..  and the big psu rack with edcors  just says  'whoa'  ..  like    respect to a big horse,  close by.

Don't even wan't to talk about the ge and rca  7591a  ..  took a couple of years to get a bunch together ...  not cheAp! I very much admire the Fischers and such  :)

Fortunately, much of this build is recycled,  odds and sods  and    improvised things over lots of years.

...

I already have  steel 24ru  wheeled rack, with a fold-back top section,  that will suit these thing(s)  well,    in the  *lounge* .

....

Looks like the limiter-amp signal unit will run to (aud) :

-> 500 for traffos, 380 quality nos tubes+sockets, 310 rack/quality vu/cabling and 200 caps/resistors/other  -> 1390

The psu would be something like (aud) :

-> 500 (traffos/choke), 70(caps), 60(regulator modules x2), 180 (case/iec filter/wiring/connectors/panel prints)  -> 810

And the 1RU eq (poorman RC eq) -> 200

The speakers have around 600 aud in them for a pair  ...  timber with vintage timber front lattice , 12" driver for use  as subs to be biamped with my old Tannoy 8" concentrics and a few amps that I have lying about ...

So thats  around 3Kaud all up,    equivalent to around 1900usd    ..  with all the fees  :p 

....    but alot of that is our low Aust dollar on the OS parts, plus delivery to this distant locale.

Its about the same as the individual pieces I've previously done .. seperately  :)

.....

Thankfully, the Trimax iron was affordable  ... similar US made would add quite a bit in today's high cost envi-rironment here in Oz. Think Kenyon and better ..  I've still to test these units, but I have high hopes.

....

On the plus side, the tubes and traffos are top quality and lovely to behold - what else matters  :D

The PSU is no compromise and remote (yay!) 

Speakers are beautiful solid wood with classic large paper coned alnico drivers.

A worthy last big build set  ...  while I continue to rationalise more and more kit  to downsizing....


 

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alexc

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It's getting there ...  I'm hoping to be wiring up as soon as winter is over  :)

I'm doing the woodwork for the subs  and the metal work at present.

............

Not expecting any surprises in this build ..  it's all well trodden ground  ...  great, historical circuits with great parts gives great results.  And the cost, well one defrays it over several years  :)  any which way.

......

Actually, the potential surprises    are  in fact :

-  can the gain cell  input transformer  .....  the front one ..  take being 'tickled' with a goodly CV ..  'time varying dc'  signal  ...  without saturating  or otherwise being susceptible to 'transient distortions' of various kinds

-  do the gain cell transformers have the required wide band freq in the planned configuration ?

- Will the gain reduction tubes I've chosen perform  'good' for this gain cell and it's transformers ?

- Does the gain cell output traffo have the where-with-all to take some 'unbalanced dc' in the primary  ..  when the spec says clearly 'no'  :(        ..  particularly at the all important transition from idle to gain reduction  ..  the 'whoomp' transient  as it were

- Is the CV dynamic  or 'time constant' easy to dial in  ..  such that it is not massively 'overshooting'  then 'chattering'  the signal to an uneasy place  ...  or conversely, can one get a fast enough attack with a smooth, 'unlumpy' release  :eek:

The answer to most of these is 'hefty iron' of the 'right kind'    ...    with 'lucky dip' vintage buys, that I especially like,  it can be a little unpredictable ...  that's why I took up the 'pedal steel', it just always sounds like a guitar with a great tube compressor.

Sometimes it just takes some damnable good luck  ..  you never can tell until you  get there ...   

OR you can just spring for the Sowters ! and be done with it  8)

...

OK I have a good stash of these tubes ...  and some less glamourous of the same type are available quite easy ....  there's a number of alternatives in easy reach ...  several russian equivalents ...  they're mostly fine  :).

The main difference here over my previous used 6k14p is the gm is quite a bit higher in these 6eh7 types. It's of marginal usefulness really ...  it's the number of trioded-pentodes per phase that is important .. more is better ..  lower Z ..  more gm  etc.

So here I have fewer (only 2 per phase) tubes, but of higher (idle) gm ..  *and* feeding a really high Z winding 100K plate-plate  :)

It's usually too low of an impedance primary and/or too low rated current *plus* early saturation with any unbalanced dc  ..  that gets you ...  resulting in too much transient distortion ...  and early bass cut  :mad:

And if it's not that .. then poor shielding will most often ruin things. 

If you get past all that, THEN decently low ripple psu rails may just possibly keep the results ..  quiet  :)

Don't even want to talk about ..  microphonics  >:
 

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alexc

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A measure of success is if you can use a stereo vari-gm limiter *without* a modern noise gate  ...  and live with the 'silence' noise floor.

I used to rely on the finalizer multi-band noise gate ..  hands down best for hifi gating, and also the yamaha digital desk companders ... which were pretty good indeed.

Then as I became more savvy building tube limiters, I found I could get away with the  modern and generic  'art tcs' gate  to tame the silent passages  ...  (no offence to ART audio - I am a fan of their engineering!)

Now I can get away with no gate .. most everything is really quiet now with just a little hiss  with very little hum  ... but big on the springy, elastic and syrupy lift that great tube compression provides.
 

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alexc

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Have downsized a bit since the heydays  ...  I still have a lot of actual hay, tho', big stacks of it  :)
 

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alexc

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Still have my guitar rig :)
 

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alexc

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A few moar things on the way, another rework  job done (an api styled eq with 3 band)  ... 

Half way thru the winter 'proofing' of last diy season's stuff .. 

My 'encel' japan refurb stereo integrated amp with pp el84  just  nails it ...  driving the tannoy 8" concentrics (system 8 circa '95 or so).

Source is a Minix media player doing mp3s and wavs and such ... with a dac to analog rca jacks

Bass is just great .. in 'spl vitaliser' speak, it seems to just find the  right balance of 'soft' and 'tight'  ... the amp is loafing along at approx 5Wrms into 8ohms average-ish. The 'silent' performance is awesomely quiet .. no hum, hiss..  nuthing.

It has a 1KHz sine 3% thd at around 12Wrms into 8ohm resistive load. 1% is around 8W and 0.7% at 5W  ... I have both a 'Volume' control and a 'nfb' control ...  makes it easier to dial in volume vs distortion.

Using the russkie 6P14P-ev with a reasonably high B+ around 390V and screens lower at 360V or so (from memory) ..  a simple R-C filter on the screens, no UL here, no,  neither zener nor glow tube.

No problems whatever driving these tannoys ..  which I used to use with a 120Wrms 8ohm fet/bjt power amp ..  for years :)

As was said in the seventies 'yes sir, it can boogie!'

.....

I have to do some comparisons,  with the el34 pp monoblocs  (which do 30Wrms into 8ohms at very low thd) on the tannoys.


For now, I'm spending days and nights with the 8" tannoys with  el84 pps and loving it  ...  just right volume  :)

....

Current winter job is cutting pine boards and finishing a new large 19" pine rack  ...  for the lounge room ....  the tube stuff just eats rack space for breakfast!.

Also 'reset' the top panel of my little 6V6 pp guitar line+speaker amp  ...  I mean tossed the old layout, cut new alu panel and now re-laying out  and drilling up    ..  really proper-like (this time). I think it's my 3 time in this format!

Noise performance is everything. Character comes second.

So - have to get moving, half way thru winter already.
 

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