Balanced Bridge Gilmore Amp
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BoyElroy

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Well, I've had the balanced bridge Gilmore running as a head amp and preamp for almost a week now and wanted to share some notes with any Gilmore DIY'ers out there.

First, there is a noticeable difference between the BB Gilmore and standard Gilmore. Used as a headamp, I'm measuring (as KG states in his project description) twice the voltage swing at output. To my ears, at least, this translates to more midrange and bass. The upper freqs. don't seem that different to me, but the entire midrange to lower bass seems stronger, with greater bass extension on my 325's. I performed the necessary surgery on my Grados with a heat gun and soldering iron and found out that its actually a pretty simple, non-damaging procedure. I'm also going to put in a standard headphone jack so I can use regularly wired headphones as well.

I'm a bit confused on using the BB Gilmore as a preamp. I've put in both balanced and single ended output jacks on my BB Gilmore but because my McCormack amp only has single ended inputs, I'm using the two positive outputs and ground in a single ended configuration. As far as I can figure, this shouldn't result in any significant change from the standard Gilmore because I'm not using the full balanced circuit, but there seem to be some important audible differences between this amp and my prior standard Gilmore. I wonder if someone could explain to me what exactly is going on here...

As for the sound of the BB Gilmore, I'm hearing a more solid bass foundation (almost like my Apogee Stages have gone from a 40 Hz low end to 30 Hz) and things are shaking in my apartment that didn't shake before. The center image is also much more strongly defined and the soundstage seems wider and more firmly fixed. The biggest difference, though, is in the midrange and mid-bass. The sound is much fuller and where the standard Gilmore had a more airy presentation, the BB Gilmore sounds more balanced from top to bottom. The first thought that went through my head was that there was more resolution with the standard Gilmore but on continued listening, the BB Gilmore has the same high resolution imaging, just more bass and midrange along with it. The high freq. information on the BB Gilmore is still there, but it is offset (?) by a more full lower end so in that sense, I guess it sounds less "airy".

I'll stop there because I don't intend this to be a real review or anything like that. I suppose the main reason for posting this info. was really to point out that for someone planning to DIY a Gilmore, there doesn't seem to be any reason not to build the BB Gilmore instead of the standard Gilmore. The cost of adding the extra boards/transistors is actually quite small and there is no need to buy anything additional. You can still use a standard stereo pot/attenuator at the input and use only the positive outputs and ground to run the BB Gilmore as a single ended amp. If you decide to run it as a full balanced bridge amp in the future, all you have to do is add another headphone jack for the headphones and XLR outputs for the preamp side.

BTW, I didn't bother matching my transistors (again!) and my dc offset at output is .001 vdc on one channel and .006 vdc on the other. I'm not sure if handmatching the transistors (as long as they're from the same production lot) is necessarily required (as long as you intend to use the opamp in the circuit, that is).

One last construction note:

This being my first balanced project, I assumed that I had to connect only the positive and negative outputs to my two neutrik jacks. The sound quality was awful; all treble and little bass. When I turned up the volume, it caused all sorts of pain and aural agony. I took the whole amp apart, re-wired everything, still no luck. I finally picked up KG's insructions and realized that I had to ground the neutriks, even though the headphone jacks carry only + and - signals. Once I connected the neutriks to ground, everything sounded right as rain again. So for anyone having treble/bass problems, my tip would be to check all your ground connections...
 
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fyleow

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Maybe I've been gone off the boards too long but what is a Balanced Bridge Gilmore Amp?

I've already build myself a regular one how do I upgrade?
 
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BoyElroy

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Hi Fyleow,

Here is KG's description of the Balanced Bridge Gilmore amp:

"The balanced bridge output version of the amplifier (figure 2) is for those headphones that can be wired as dual mono (see the addendum for instructions on converting a pair of standard Grado SR-80 headphones into dual mono headphones). It has twice the voltage swing, twice the slew rate and 4 times the output power (competes with the $2600 HeadRoom balanced Max amplifier). "

Basically, this means that you can externally re-wire your Sennheisers or internally re-wire your Grados to run in dual mono mode with the balanced bridge Gilmore. If you already have the standard dual Gilmore board, all you have to do is run off another dual Gilmore board and populate it with 2 negative output stages (1 for each channel) as shown in KG's Headwize Project library. You can use your existing dual board as the 2 positive legs and use the new dual board as the negative legs.

If you're building everything new, it may make more sense to keep the channels separate; that is to say, putting one positive stage and one negative stage side by side on the same dual board

Here's the negative output stage you have to add (you can put two of these on one board). To keep the wire leads short, you may want to consider sandwiching the two dual boards back to back. You do not need to fully populate the new board; just the parts inside the blue circle.



One important note regarding opamp connections: Although KG's diagram shows the neg. audio output feeding back into pin #3 (+ in) of the opamp, doing this pushed my dc offset up to .3 Vdc. Connecting the neg. audio output to pin #2 (- in) brought the dc offset back down to .001 vdc. So please keep in mind that you will then have to connect the positive audio output to pin #3 of your opamp (+ in).

Having done this, you now have the choice of running your Gilmore in a balanced bridge dual mono mode using the two (+) audio outputs and 2 (-) audio outputs or single-ended mode using the 2 (+) audio outputs and ground. You can also change back and forth between the two depending on how you switch things.

After devising a new rapid cable swapping method to AB the 2 modes, I wrote up the following revised observations in a response to a question by Judo over at Headwize:

"that said, the balanced bridge (BB) Gilmore (used as a headphone amp) is a whole different beast from the standard Gilmore. It has more bass, yes, but from top to bottom, it is razor fast and reproduces transients with a speed and edge like you would not believe. I rigged up a couple of cables, one standard plug and dual mono pair, for my Sennheiser HD25-1 headphones (the nice ones) and I've spent the evening and early morning AB'ing the single-ended output and balanced output. The BB Gilmore radically changes the character of the HD-25-1's; the soundstage expands, the highs go way, way up and the dynamic range of the headphones opens up until the standard Gilmore actually sounds a bit plodding and closed-in in comparison. That's not to say the standard Gilmore sounds bad, but its not nearly as fast and agile as the BB Gilmore.

But that raises another issue; it's clear that the dynamic range of the amp expands greatly from high to low, as does resolution, channel separation and soundstaging. But this razor fast speed and transparency also emphasizes higher freq. transients and some cd tracks may sound too bright for some peoples' tastes. And when I say the highs are extended, boy, are they extended. In this regard, even though the BB Gilmore has greater bass extension, it's so fast (and different in this regard from other amps that I've experienced) that some people may actually prefer the bass of the slower standard Gilmore. The BB Gilmore, if I may say, is like the standard Gilmore on speed and andro. It really brings the HD25-1's and my Grado 325's to life; maybe a bit too much life for some. This makes me think, though, that this would be an ideal fit for a pair of more sedate(?) headphones like the Senn HD600's.

But to get back to my main point, if you build the BB Gilmore, you can have your cake and eat it, too. You have the choice of using either single-ended outputs or balanced bridged outputs so if you don't like the sound of the balanced bridged output, you can easily switch to the single-ended output and run it basically as a standard Gilmore."
 
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Dreamslacker

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Quote:

Originally posted by BoyElroy

One important note regarding opamp connections: Although KG's diagram shows the neg. audio output feeding back into pin #3 (+ in) of the opamp, doing this pushed my dc offset up to .3 Vdc. Connecting the neg. audio output to pin #2 (- in) brought the dc offset back down to .001 vdc. So please keep in mind that you will then have to connect the positive audio output to pin #3 of your opamp (+ in). Both neg. and pos. outputs feeding back to the opamp should be connected to ground with a .33 mfd cap.


Hi BoyElroy.
Are you saying that the negative output and postive output of the amp should connect to the opposite inputs of the opamp than in the schematics?
And both inverting and non-inverting inputs of the opamp are to be coupled to ground with 0.33uF caps?
 
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BoyElroy

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"Are you saying that the negative output and postive output of the amp should connect to the opposite inputs of the opamp than in the schematics?"

Hi Dreamslacker--

Yes. When I followed the schematic, it increased the dc offset considerably to .30 vdc. The dc offset with no opamp in the circuit was .114 vdc. Changing the connection reduced the offset vdc to .001 vdc and .006 vdc, respectively.

Here is a diagram of the revised opamp wiring and complete signal and ground wiring I used for my BB Gilmore.



I found this offset nulling circuit on the op27 datasheet and it seems to show the (-) signal going into pin #2 and the (+) signal feeding into pin #3, which again is the opposite of KG's schematic. Maybe Kevin Gilmore could clear this up?

 
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Excellent material! Thanks for leading the way, and the review.

This woz the direction I intended to go after building the std amp (currently awaiting delivery of kit from antness). So thanks for doing the pioneering. And thanks to Kevin for an excellent design.

After building a bridged version, I would assess the merit of building another PS, to end up with a PS per channel. Do you think there is any mileage in that idea?

Steve
 
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BoyElroy

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Hi Steve,

I think that's how the Headroom Blockhead Max works, doesn't it? From what I remember, it's a full dual mono design up to and including separate ac cords. I'm definitely not an expert on power supplies but from what I understand, it seems to be generally accepted that you can never have too good a power supply, especially if you have a circuit like the Gilmore amp that's capable of reflecting those improvements. I think that the standard psu that Antness provides is capable of excellent sound, but of course it's made to fit a certain price point so, yes, I think you could definitely improve on it (such as using discrete schottky rectifiers and higher quality caps). Using a full dual mono psu is something I'm kind of considering as well...
 
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Dreamslacker

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Another question:
Did you bypass the opamp with the capacitor across the -ve input & output or across the +ve input & output instead?

I would be building the KGCA on perfboard so I can't quite make out the circuit board design. I'd better understand schematics.
Nice work, btw.. Any pictures of your amp?
 
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BoyElroy

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Hi Dreamslacker,

I just re-drew some stuff on the drawing above...I don't know if that helps or not, but I should be able to put up re-done schematics later this afternoon.

Unfortunately, I don't have a digital camera right now. Maybe I'll borrow one from work...
 
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Quote:

Originally posted by BoyElroy
Hi Dreamslacker,

I just re-drew some stuff on the drawing above...I don't know if that helps or not, but I should be able to put up re-done schematics later this afternoon.

Unfortunately, I don't have a digital camera right now. Maybe I'll borrow one from work...


Hmmm... It didn't really help relative to my question but I've figured it out now. Had to refer to my drawings for the opamp layouts when I did my Proto42 amp.

So the bypass cap is still across the inverting input and the output of the opamp.

However, you don't seem to have coupled the inverting input of the opamp to ground with a capacitor in your drawing. Or is it that you did not find it necessary?
 
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BoyElroy

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Hi Dreamslacker,

I'm sorry I couldn't give you a better answer this morning; I was in a bit of a rush. Here's a variation on KG's "Negative Output for Balanced Headphone Amplifier (one channel)" schematic from the Headwize site.

I can't say if the method I used is the correct one and I don't know if my dc offset shot up using the original schematic configuration because the components were not closely enough matched, but the re-wired method in the below diagram does work in decreasing the dc offset considerably. My one concern is that it may be adding noise back into the current input of the dual FET.

Regarding not coupling the inverting input to ground, no, I didn't find it necessary since the dc offset went down and stabilized at .001/.006 vdc. Again, I wish I could get KG's input on this issue...

In any case, here's the schematic with the changes I made to the opamp inputs.
 
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BoyElroy

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Hi Dreamslacker and/or anyone else who's interested,

Here's a neat little diagram that I found in an old electronics handbook showing various opamp input/output combinations. Please correct me if I'm screwing this up, but it seems that in the original schematic with the + signal feeding into the opamp's inverting input, you'd basically end up with figure "D" below.
 
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Dreamslacker

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Quote:

Originally posted by BoyElroy
Hi Dreamslacker and/or anyone else who's interested,

Here's a neat little diagram that I found in an old electronics handbook showing various opamp input/output combinations. Please correct me if I'm screwing this up, but it seems that in the original schematic with the + signal feeding into the opamp's inverting input, you'd basically end up with figure "D" below. If you feed it into the non-inverting input with the - signal going into the inverting input, you'd end up with figure "C". I hope this makes sense. I don't have any formal training in electronics and I'm trying to reason this out as best as I can!


That's odd. Feeding the positive output of the amp into the inverting input of the opamp should be correct.

If the amp drifts +0.3v, feeding that into the non-inverting input of the op-amp will result in an output of +0.3v (assuming gain = 1).
However, fedding that into the inverting input will result in an output of -0.3v. That should cancel out the drift. So it seems odd to me.
 
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BoyElroy

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Hi Dreamslacker,


Boy, that was a fast response...I'm sorry but I kind of realized I wasn't making much sense and I edited my comments, retracting my nonsensical conclusion (ooops, too late).


It is odd. The dc offset measurements I took using the original configuration seemed indicative of phase re-inforcement. As I mentioned above, the dc offset with no opamp in circuit was about .14 vdc or so. With the opamp in, the dc offset increased to .3vdc.
 
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kevin gilmore

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Hey dreamslacker, you have sent me a PM with your PM turned
off. Send again to my email address...
 
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