[antomas]

... to say heartfelt thanks to Dr. Kevin Gilmore.

Massimo

 

[aeberbach]

The only thing a relay gets you is short signal path - but you can achieve the same by moving the selector switch back to where the signal enters and using a coupling and shaft to bring the control to the front panel.

 

[BoyElroy]

"... DC coupling at the output. I think it would be a good idea to study the circuit of the power amp you expect to connect at the output of the GD. "

Antomas,

Thanks for pointing that out. I guess it would be a good idea to measure the source output dc voltage BEFORE connecting the preamp and amp. I think I just dodged a bullet on that one. As soon as I get home, I'll take a DMM to my cd player output before turning on my Gilmore and keep my fingers crossed that it comes out to something harmless.

I wonder if Kevin Gilmore has worked on this dc coupling issue at all...



aeberbach,

Your solution really seems the simplest and easiest to implement, short of having no switch and only a single source (which may be my preferred option).

On the other hand, the parts for the actual amp itself are so cheap it may make sense to take Antomas' approach of building 2 separate amps for 2 sources and simply switching one or the other amp on and off. At least that's how I'm understanding Antomas' design approach. I hope I'm not messing up your idea here...

 

[Doh]

Here is a link for a preamp using simple rotary switches to change inputs:

http://www.diyaudio.com/forums/showt...5&pagenumber=1

Seems to work quite well at the expense of a little more complex wiring.

With regard to DC coupling, it's been my experience that most commercial gear uses capacitors extensively to block DC. In taking apart my NAD L40 (an all in one unit) I found capacitors at the output of the CD section, at the input of the preamp section, and at the input of the amp section. After thinking about it, I decided a whole bunch of them were unnecessary and replaced them with jumpers to quite good effect.

BoyElroy,
I bet that if you take the covers off your source and amp, you will find both output capacitors at the source and input capacitors in the amp to block DC. In short, I don't think you did any harm to your equipment by hooking up the KG amp as a preamp.

 

[antomas]

Doh,

I agree with you, most (if not all?) equipment use capacitors everywhere, so half of them are useless. I didn't want to terrify our friend BoyElroy, just suggest to check, for the peace of mind, the schematics of his power amp before connecting the Gilmore (I'm a well-known paranoiac...).

BoyElroy,

I think my idea of 2-amps-in-a-box makes sense only if you have 2 simultaneous listeners in the same room (say you and your partner), either listening at different sources or listening at the same source with two headphones, therefore needing separate volume controls.

Thanks you all,
Massimo

 

[BoyElroy]

Hi Guys--

Thanks for your help here

. Now that I've stopped hyperventilating, I've got another idea to run past you; what do you think of using a bridged version of the Gilmore dynamic amp as a pre-amp? Goon-Heaven mentions in an earlier post the idea of using a bridged Gilmore as a power amp for speakers. Even as a preamp, the advantages of the bridged Gilmore (twice the slew rate, twice the voltage swing, dual mono operation) seem to be worth the marginal extra cost involved...any opinions?

BTW, I think I'll build my new Gilmore preamp using all Beyschlag resistors from Digikey. The cost seems pretty reasonable and I like their accuracy. I hope they sound okay

 

[Doh]

Hmm... I've thought about this one, too.

My main issue with this is that I really do want to use this thing as a headphone amplifier as well as a preamplifier, and am not too keen on the idea of dissecting apart a pair of grados to use in Dual mono.

The upshot of going with a bridged amplifier, are that

1) I believe this means that the preamp may be set up for balanced inputs
2) the greater voltage swing brings the voltages up to a range where one might consider building a simple current follower to boost the wattage of the thing to the point where speakers may be more easily driven.

I will be building my KG amp from Beyschlag resistors from Digikey as well. I just wish I knew when my boards from antness will arrive.

-Doh

 

[antomas]

Hi there,

As far as the bridged amp is concerned, I think it would be a very overkill to use it as a preamp. If, instead, you want to use it to drive some VERY efficient loudspeaker (Klipschorn?), that could be very nice. In any case, make some computation to understand how loud the combination of bridged Gilmore and hi-eff amp could sound (there must be some formula around here for headphones, just use Watts instead of mW).

I tend to remember that Dr. Gilmore suggested somewhere to add more output transistors to drive 8 ohm and less. I caressed for a while the idea of adding more transistors on one of my two (future and non-bridged) amps, to be able to experiment with my (very far in future) next DIY experiment (maybe a horn speaker). Maybe I'll just put some additional trace in the amp board.

By the way, I recently heard a terrible discussion on the tnt-audio forum (in italian) about a similar subject (if a 3W tube amp connected to a horn loudspeaker may or may not sound loud) and I've seen that audiophiles may hold very religious opinions about this kind of things.

If you're really interested to the idea, since I think you're not religious (in the audiophile sense) I'd suggest you to get a $49.99 Sound Level Meter (SLM) from Radio Shack (I got one one week ago) and to measure, using your favourite record and favourite volume, the max Sound Pressure Level (SPL) from your loudspeakers. Then compute the SPL you could expect with 4.5W of the bridged Gilmore or 1W of the plain one (not sure about the figures). Then lower (or raise!?) the volume knob until you read on the SLM that figure. This not to have an idea of how good it could sound (I suppose better), but just to have a very rough idea of how loud it could sound. In case, the Radio Shack code (the toy is getting out of stock) is 33-2055. Certainly not a professional tool, but useful for rough measurements.

I suspect Jude will ban us from the forum after this very OT discussion...

Massimo

 

[antomas]

For the resistors I'll go with the Beyschlags as well.

I still have a problem in finding one of those nice plastic (non shorting) locking headphone jacks from Neutrik. If someone could be so kind to find and send me a pair of these, I'd be very glad to send him/her a couple of records thru cd-universe...

There must be a curse on the european electronic components. Why do we have to purchase them in USA?

Massimo

 

[aeberbach]

boyelroy, I can't take any credit for the idea. It's an old practise and you can buy suitable couplings and shafts at places like angela.com or hndme.com. I'm using one to drive the stepped attenuator in my Gilmore amp to minimize wiring (and just because I had one that wasn't being used).

 

[Doh]

Here's some info about the Beyschlag resistors:

0.4 Watt "5043ED Series" aka SFR25:

http://www.bccomponents.com/Uploads/...ts/sfr1625.pdf

0.4 Watt "5063JD Spacemeiser Series" : can't find entry in cross reference table. Perhaps SFR16S? (these have worse noise ratings than the SFR25 if you read the datasheet)

0.6Watt "B0207C-F Series" aka MBB0207-50:

http://www.bccomponents.com/Uploads/...68_leadpro.pdf

It seems that, at very least, the 0.6 watt variety uses steel endcaps. No mention in the datasheet as to the type of endcap the 0.4 watt resistor uses, but I wouldn't be surprised if they were the same.

Audionote UK uses Beyschlag resistors in their equipment. Their comment on these resistors is that they are slightly magnetic, yet sound pretty good.

I don't know what to make of it, but thought someone might find the information useful and be able to shed some light on it.

P.S. Just for kicks, I cracked off the epoxy coating off one Vishay RN60 resistor. I can't tell for sure, but looks kinda like the end cap is made out of aluminum. Kinda interesting to see the internals of one of these tings. You can see where the laser has cut a spiral into the film deposited on the ceramic core. Oh well. I think I've played around enough with this resistor business for today.

Just a though...it seems that these manufacturer's make a point to mention when their resistors have steel endcaps. I wonder if that implies that the cheaper beyschlag resistors don't have the steel endcaps found on their more expensive brethren. Only one way to find out, I guess...

 

[antomas]

Hi Doh,

thanks for the interesting report and... sorry for your resistor.

My other candidate for resistors was Meggitt/Holco; thye should have copper caps. No idea about what's better. Should maybe steel be more stable than copper in the long run...?

Ciao,
Massimo

 

[Jupiter]

Quote:

Originally posted by antomas My other candidate for resistors was Meggitt/Holco; thye should have copper caps.

The new Holcos have steel caps, but you should still be able to find the old Holcos. Anybody know if Mouser is selling the new or the old type? (I emailed Mouser, but never got a reply.)

 

[Doh]

The problem with Holco is that they tend to be rather fragile as things go. The older production Holcos have copper leads and copper endcaps. When I used these to build a CMOY amp a while back, I was impressed by their warmth and fullness of sound compared to generic metal film resistors (most certainly having magnetic endcaps).

The newer Holcos evidently do not sound as good, though I have not confirmed this myself. Due to their mechanical fragility, Holco replaced the copper endcaps with steel endcaps. The newer resistors may have steel leads as well.

For industrial uses, I would bet that steel is preferred for its durability. For DIYers like us, I'm not sure its so important. (except perhaps in the case of Holcos, as some have reported that the resistors become faulty after bending the leads too close to the resistor body)

Another possibility is Roederstein Resistas. One source in the USA is www.welbornelabs.com. They are about $0.20 a piece, and it appears that welborne will no longer be carrying them in favor of the vishays. Michael percy stocks the resistas as well.

Would anyone with Beyschlag resistors on hand care to perform a magnet test with them, just for the sake of satisfying curiosity?