Jump to content

The "Ultimate Gem" SEP is here!


tube fanatic

Recommended Posts

The Little Gem SEP which I featured on here some time ago quickly turned into my most popular amp design.  From the original to every variant (using different tubes) built by me and others, it has received rave reviews due to its extremely high performance to parts cost ratio.  Although it's in the flea-power category, in most systems located in small to medium size rooms using Heritage and Reference series speakers, it is capable of playing far louder than anyone cares to listen.  Yet, there are some guys who just won't trust their ears and have asked for a version with somewhat higher power and the ability to tailor the amp's sonic characteristics to suit specific systems.  I like challenges.........

In the thread on the Little Sweet Potato I discussed the development of tubes which were designed to be directly driven by the high output phono cartridges typically used in budget phonographs in the late 50s/early 60s.  The output tube used in the Ultimate Gem, the 50HN5, was a fairly late addition to that "family" as it was registered by Westinghouse in 1964 (to compete with the solid state amps coming into use which provided somewhat higher power output).  The way the tube is configured here, it's providing an output of about 1.7 wpc @ 1 kHz.  Trust me when I say that you absolutely don't need more power unless your listening levels are extremely high, or your room is large.

As to features, I placed the variable high frequency filter (aka "ear bleed filter") externally to allow ease of adjustment for specific rooms and listening tastes.  It turns out that the amp is so inherently smooth that I've kept it at its minimum rotation even when playing recordings with large amounts of high frequency boost. Regarding the bottom end response William pointed out, after building the original Little Gem, that the bass output was too much for his system (using La Scalas).  Since the amount of negative fb isn't terribly critical, in this amp I made it adjustable (my prototype has the adjustment internally located; it can just as easily be made externally adjustable).  It allows the feedback filter corner frequency to be set between about 36 Hz and 60 Hz. That range of adjustment should cover just about any possible system requirements.

Like the original Little Gem, this amp uses a separate power supply to keep AC line related issues far from the amp.  The p/s is connected using some standard #18 line cord and Anderson Powerpole connectors which I've discussed before.  You can locate the p/s anywhere from one foot to a few feet away from the amp chassis.

I wasn't prepared to see the linearity this amp produced on the 'scope.  See the pics below of the 320 Hz, 1 kHz, 3 kHz, and 5 kHz square waves.  I've also attached pics showing the action of the "ear bleed" filter at 5 kHz.  It is very effective!  The sound of this amp actually surpasses that of the Little Gem in regard to bass output, sound stage size, and sound stage depth.  I can't explain this.  Perhaps it's due, in part, to the slightly larger output xfmrs or other variables.  But, whatever is the cause, no one is complaining!

The best part of this whole concept is that you can build the power supply shown on the schematic and use it for the Little Sweet Potato, original Little Gem, or the Ultimate Gem!  Someone interested in starting out with DIY amp building can begin with the simplest, the Sweet Potato, move on to the original Gem, and then the Ultimate Gem if more power is needed.  Cost is greatly reduced by having a common power supply.

The schematic is attached below.  Please contact me if you spot anything which doesn't look right!!!

Obviously, I advise that you DO NOT BUILD THIS OR ANY OTHER AMP WHICH USES LETHAL VOLTAGES IF YOU DO NOT HAVE PROPER EXPERIENCE!  Seek the help of someone who does if you have doubts.

Maynard

post-40520-0-15860000-1454459120_thumb.j

post-40520-0-55100000-1454459138_thumb.j

post-40520-0-27260000-1454459156_thumb.j

post-40520-0-87540000-1454459179_thumb.j

post-40520-0-46380000-1454459212_thumb.j

post-40520-0-36180000-1454459227_thumb.j

post-40520-0-85180000-1454459262_thumb.j

post-40520-0-51700000-1454459286_thumb.j

post-40520-0-94980000-1454459308_thumb.j

ULTIMATE-GEM-2-2.pdf

  • Like 2
Link to comment
Share on other sites

William, you are correct about the high frequency filter.  C3 and C4 can be omitted as well if desired, or reduced in value to maybe 680-1000 pf.  The reason that I suggested removing only the pots on the schematic is that the caps across the volume control keep some of the ultrasonic stuff, which sometimes comes in with digital sources, out of the amp.  It's an easy matter to experiment both ways to achieve the desired sound.  No offense taken about our age!!!  At least we're still here to complain about it.......

Maynard

  • Like 1
Link to comment
Share on other sites

Paul, what is it you are looking for at 20 Hz?  I can't imagine you want to see a square wave at that frequency!  Right now, the amp is at a friend's house being evaluated.  When I get it back, I'll post the square waves at 10 kHz and 20 kHz for you.  The filter will be partially engaged as the 2200 pf caps are soldered across the volume pots.  Remember also that the output xfmrs are not considered "hi-fi" and their frequency response reflects that.  I refer you to George Anderson's review of the 125CSE so you can get an idea of what that series is capable of:  

http://tubelab.com/articles/component-testing/budget-output-transformers/

Maynard

Link to comment
Share on other sites

That is really well stated William!  I agree completely with your viewpoint.  In fact, when I have demonstrated amps for younger listeners (often in their 20s or 30s), they absolutely loved the sound of units having a fairly flat response from around 40 Hz to 12-13 kHz (with definite rolloff beyond those limits) and claimed that the SS receiver (with a huge bandwidth and vanishingly low distortion) which I used for comparison sounded positively awful.

Maynard

Link to comment
Share on other sites

Hello everyone I am new to the forum here :^)

 

Wouldn't testing an amplifier with square waves at 20kHz not give a good indication of it's response at 20kHz? That would be testing far above what an audio transformer is designed for, really you are starting to get into the upper frequency region (>200kHz) where the transformer designers place the resonant peak.

 

-Cindy

  • Like 1
Link to comment
Share on other sites

Hello everyone I am new to the forum here :^)

 

Wouldn't testing an amplifier with square waves at 20kHz not give a good indication of it's response at 20kHz? That would be testing far above what an audio transformer is designed for, really you are starting to get into the upper frequency region (>200kHz) where the transformer designers place the resonant peak.

 

-Cindy

Welcome to the forum Cindy!  I agree with you.  In tube power amplifier stages I really don't want to see anything happening up in that frequency range as it would make me very concerned about parasitic oscillation issues.  I believe the gentleman is a solid state advocate which may explain his interest in such a high frequency bandwidth.  Perhaps he'll come back in and chat about it.

Maynard

  • Like 1
Link to comment
Share on other sites

A bit of a rhetorical question since I doubt I'll be doing a tube amp.  (though it crosses my mind every now & then)

 

Take this amp for example... (though there might be a different one that would be better to pick for my question for all I know)

 

If someone was biamping and crossing to the HF at say, 400 Hz, would/could/should you make any changes to this amp to incorporate that difference to increase its performance?

Link to comment
Share on other sites

Another gorgeous build Maynard! Betcha it sounds great. 

 

Guys, I tell ya true, for their cost and the given the simple caveat that efficient speakers are needed, these things are spectacular. 

 

I know, I've done two projects with Maynard and am mightily impressed. And he's a hell of a good guy too.

Link to comment
Share on other sites

Your guts are a thing of beauty as usual.  :emotion-41:

 

One of these days, somebody should build a tube amp where the wiring and caps, etc are on the outside, and the tubes and PS are inside, all with a fan to cool them and some protection from shock for little fingers...

Edited by mustang guy
Link to comment
Share on other sites

One of these days, somebody should build a tube amp where the wiring and caps, etc are on the outside, and the tubes and PS are inside, all with a fan to cool them and some protection from shock for little fingers...

 

To me, that's the beauty of a tube amp, with the tubes on the outside glowing. Why shove it in a black box with a fan and have look like a pro sound amp? 

 

Concerned about the device shocking little fingers? Put it out of reach...

Link to comment
Share on other sites

A bit of a rhetorical question since I doubt I'll be doing a tube amp.  (though it crosses my mind every now & then)

 

Take this amp for example... (though there might be a different one that would be better to pick for my question for all I know)

 

If someone was biamping and crossing to the HF at say, 400 Hz, would/could/should you make any changes to this amp to incorporate that difference to increase its performance?

Cindy answered the question quite well in the other thread (I guess Carl isn't around today to move it over here).  Essentially, there's no disadvantage to using a full bandwidth amp over a limited range of frequencies (it's more of a concern when dealing with the opposite- asking the amp to reproduce frequencies which it isn't comfortably able to handle).  As was pointed out, removing the need to reproduce the bass frequencies definitely allows using less costly output transformers; however, some of the less costly units will add more distortion than the better ones.  Regarding the high frequency response, a lot depends on whether you are looking for flat reproduction all the way out to 20 kHz.  If so, the less expensive offerings may fall somewhat short (although in my opinion, anything which provides an essentially flat response up to 12-13 kHz will be just fine).  Also, much depends on the type of crossover you are using for the biamping and the slope of its filter.  If I personally were biamping or triamping, and using the tubes only for the midrange driver or tweeter, I would make no changes to this or any of my other amps.  

 

Thanks for the praise Marty!  It's nice to see you back on here.  Don't be a stranger!!!

Maynard

Link to comment
Share on other sites

Hello Maynard, I want to say I think it is a great thing you are doing. I like the DIY nature of your threads, you aren't trying to sell anyone anything and it is such a breath of fresh air. I hope I wont be too much of a bother if I ask you a few questions.

It looks like you use a separate volume control for each channel? What is the reason for the deviation from a standard stereo pot?

Now a couple power supply questions. I am used to seeing many more filters in power supplies. In your experience have you found that tube related equipment doesn't benefit from said filters? I am thinking todays digital logic circuits require more care in regard to noise reduction.

-Cindy

  • Like 1
Link to comment
Share on other sites

It looks like you use a separate volume control for each channel? What is the reason for the deviation from a standard stereo pot?

 

precise control over the sound stage. i have one of his amps and i was initially against the dual-pots, but they grew on me and i find myself tweaking each knob for granular control just as he said i would. 

Link to comment
Share on other sites

Hello Maynard, I want to say I think it is a great thing you are doing. I like the DIY nature of your threads, you aren't trying to sell anyone anything and it is such a breath of fresh air. I hope I wont be too much of a bother if I ask you a few questions.

It looks like you use a separate volume control for each channel? What is the reason for the deviation from a standard stereo pot?

Now a couple power supply questions. I am used to seeing many more filters in power supplies. In your experience have you found that tube related equipment doesn't benefit from said filters? I am thinking todays digital logic circuits require more care in regard to noise reduction.

-Cindy

No bother to ask as many questions as you like!  Michael's description is right on target.  With a single, stereo, volume control there's no way to compensate for the usual imbalance in recordings, room acoustics, and our ears (unless the amp happens to have a balance control- a very rare presence in modern equipment).  I find it very annoying if I sit down in the sweet spot and the whole soundstage is shifted, say, 30 degrees to the left or right.  It annoys me to the point of not being able to enjoy the music.  Contrast that with having a totally symmetrical sound stage extending equally to either side of the 2 speakers with every recording (and if it's a mono recording you can create a perfect center image).

Regarding the power supply filtering, we don't have to get as crazy with ripple as when using other types of circuits, as you stated.  Since you have a math background, and have worked in p/s design, I'll give you some technical info:  In this amp, the ripple at the input filter cap is about 0.4%, and at the first output filter cap, which feeds the plate/screen, it's only about 0.004%.  The driver is running on basically pure dc.  This makes for an extremely quiet amp in spite of the very high gain of the output tubes.  Given how inexpensive high value electrolytic caps are, I rarely find the need to use one or two chokes which could increase the cost of the amp by $50-$60.  In a budget design like this, I consider that to be an unnecessary expense.

Maynard

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...