[Glassman]

I'm going to make another Gilmore, this time the fully balanced version, but I'd like to use different transistors in place of the four parallel 2SA1015 and 2SC1815 on the output.. I want to substitue them with just one piece. I found 2N2222 and 2N2907 which are 600mA in contrast to 2SA1015 and 2SC1815 which are just 150mA. and 250+MHz Ft in contrast to 100MHz.. should I use them in the third stage??

many thanks!

 

[ppl]

the reason for paralelling manny low current transistors is not just to obtain more output current it is also to allow each transistor to only handel part of the load and thus keeping it's Current gain at it's higest. since Hfe drops after a nominal amount of current is reached one transistor even one with a higher current rating will often times have less Hfe for operating currents the load uses.

another reason is Power disipation in each transistor. since Both types of transistors the 2N2222A and the type Kevin uses are rated at the same power also since the heat is spread out over more area thermal management is less of a problem

 

[Arzela]

If memory serves, Sijosae over at headwize
has built, and posted schematics, for
a modified Gilmore using one output
device per channel.

 

[Glassman]

sijosae used MJE15030 - 8A, 150V, 30MHz.. I think this is not a good idea - I want something like 2SA1015 but with more current and the more Ft the better I believe.. what about BD140 & BD139? They are used frequently in audio here, are they sufficient for Gilmore? for what particulary should I look into the datasheets to find the right substitute for Kevin's trannies? they are not easily obtainable here in Czech

thanks

 

[Squalish]

If you want more current, simply add more output transistors, and adjust the bias(via the LED) and the gain(via the feedback resistor) to taste. I'm not exactly sure about the math of biasing the transistors to different current ratings, or that combined with more transistors, but I'm told that you can change the current going through them with a simple change of the LED.

 

[Glassman]

in fact I don't need that much current as I don't use any low impedance headphones and when I'll use Gilmore in its balanced version, the current is doubled anyway.. what I want is some good substitute for Kevin's trannies from the 2N.. line.. I just looked at some datasheets and found that 2SA1015 has really nice flat current gain graph - is that why its used in the amp? becouse I found nothing special on it, it is very common transistor with just 80MHz of Ft so why Kevin picked this one

 

[ppl]

when i made Kevins amp i used on semiconductor MPS-8099 and MPS-8599 worked great http://www.onsemi.com/pub/Collateral/MPS8098-D.PDF

BTW Balancing an Amp will not put out twice the current it will put out half the current with twice the slewrate and 2 times the output voltage resulting in twice to 4 times the power for the same load impedance. damping factor will also be half since the two amps are now in series resulting in twice the output impedance. notice that the LM4916 puts out 14 mW stereo and 85 mW BTL mono.
http://www.national.com/pf/LM/LM4916.html

 

[Voodoochile]

Were those transistors a straight swap ppl, or do you need to alter any other values to support them?

Asking because the original parts spec'd are difficult to get in a decent grade, or even in any grade that matches the complimentary part's grade.

 

[Arzela]

Are the MPS graded? I thought
this was done mostly by Japanese
manufactures?

 

[Voodoochile]

Even better if they are 'unigrade'! If they weren't I still could not imagine it being more difficult than getting a good supply of the original parts all in one grade.

If the ppl-suggested part is a drop-in replacement... might have to pick some up to try.

 

[Arzela]

I planned to build one using MPS, but after
I discovered they have different pinouts than the Toshibas', I ditched the
project and gave the trannies to a
fellow head-fier.

You can buy them here BTW:

http://tech-diy.com/store.htm

 

[Glassman]

Kevin used them in his prototype, but he says they are discontinued, so he used 2SA1015 and 2SC1815 instead..

but again: what should I look for in the datasheets to ensure functional and quality-wise compatibility with Gilmore amp??


PS: looked at my parts store and found nothing like those MPS

 

[Glassman]

Quote:

Originally posted by ppl BTW Balancing an Amp will not put out twice the current it will put out half the current with twice the slewrate and 2 times the output voltage resulting in twice to 4 times the power for the same load impedance. damping factor will also be half since the two amps are now in series resulting in twice the output impedance.

what does this mean? I think Kevin designed this amp mainly for his Grados ie. low impedance phones, but my phones are 120Ohm and in the future I will try to get HD580/600 so 300Ohm load. does this mean that the higher impedance of the amp is not a problem for me and that I will be enjoying the extra voltage swing needed for high impedance phones? and what is the effect of the damping factor? does that mean 'not so strongly controled'?

thanx

 

[Squalish]

Quote:

Originally posted by ppl when i made Kevins amp i used on semiconductor MPS-8099 and MPS-8599 worked great http://www.onsemi.com/pub/Collateral/MPS8098-D.PDF BTW Balancing an Amp will not put out twice the current it will put out half the current with twice the slewrate and 2 times the output voltage resulting in twice to 4 times the power for the same load impedance. damping factor will also be half since the two amps are now in series resulting in twice the output impedance. notice that the LM4916 puts out 14 mW stereo and 85 mW BTL mono. http://www.national.com/pf/LM/LM4916.html

In the case of the gilmore, doesn't the current at least stay the same if not double, since there are twice as many trannies?

The article says this:
Quote:

It has twice the voltage swing, twice the slew rate and 4 times the output power (competes with the $2600 HeadRoom balanced Max amplifier).

Oh, and does slew rate == voltage swing?

 

[ppl]

Kevin's Amp is allready a High Current Design so this would be no problem to operate the amp in the Bridge mode with headphones of about 25 ohms or Higher. Damping factor is also not a problem with this Amp in the Balanced mode.

My reason for posting at all was to correct the misimfrmation that Bridging Doubles the available output current, when in fact a bridge Amp will have half the Available output current of the same Amp operated in the Bridge mode. This is true with All Bridged Amplifiers. search BTL or Bridge to Load Audio Amps on the net for futher info.

Kevin's Amp will have no problem driving your headphones in the Balanced mode and will provide plenty of Damping factor for your Sen's.

Speaking of damping factor, This is a term used in Loudspeaker Amps it is the ratio of the Amplifiers output impedance vs. the Load impedance. since each half of the Bridge Amp is in series with the load its output impedance will thus be added to the total output impedance.

Again since Kevin's Amp has a low output impedance to begin with this is not an issue when driving headphones. If this Amp was connected to loudspeakers then the Damping factor could be a problem, However this Amp is not intended to drive loudspeakers it is intended to drive headphones.