The Ultimate DIY Part 2 ? The KGITSOJC

[kevin gilmore]

So after a successful T2 project, it is time to tackle something i've been playing with

on and off for quite some time. The KGITSOJC

The Kevin Gilmore in the style of John Curl preamp.

Now if you want to read over 700 pages of babble and bullshit, you can go over to diyaudio

and read the two threads. But suffice it to say that the Blowtorch circuit sounds very good, but

JC is of the opinion that only 2 people in the world are capable of building and getting the thing

to run. I kind of disagree with that because i have one exact copy of the blowtorch amplifier

running right next to my version.

He has some reason to say this, parts matching is ultra critical, including matching 4 resistors

to better than .01%. Not something a DIY type would want to do. And besides the unbalanced

to balanced performance of this amplifier leads something to be desired. Plus there are 2 opamps

directly in the signal path. Its time to make a couple of changes.

I've made a better version. Critical parts matching is a thing of the past. And the servo is outside

of the signal path. Performance in the unbalanced to balanced mode is much better. And the sound

is absolutely identical.

Unlike the T2 however, the rest of the design is not as obvious, nor is the way to build it.

To save space on the back panel I could just do balanced inputs, then for unbalanced inputs

use the neutrik unbalanced to balanced converters. What do you think about this?

Or would you prefer 4 unbalanced inputs and 2 balanced inputs?

In which case what kind of rca input blocks should i use?

I'm going to stick with the neutrik nc3fdh for the balanced, they are way easy to use.

I've been talking to ti khan about integrating his attenuator project into this.

What do you think about this, or do you people prefer some kind of real attenuator?

(rk50 et all)

Would anyone be interested in stacking more than 2 channels of this.

Say 8 channels for the ultimate in surround sound preamplification?

Before i start laying out the board, i kind of need to know what people really

want.

[deepak]

Kevin

Consider me very interested.

I would prefer 4 unbalanced and 2 balanced inputs, IMO XLR/RCA adapters are not something I'd really want to use...

As for the attenuator whatever is the most transparent. Only interested in 2 channels.

And an optional phono board for LOMC maybe? (only half joking)

-Deepak

[MASantos]

For those not wanting to read over 700 pages of info, could you sum up the main characteristics of this amp? Circuit, transistors used, advantages/disadvantages of this circuit?

[spritzer]

As Kevin knows, I'm in for the full on crazyness.

I'm always partial to RCA inputs over XLR's and Vampire would make the most sense here. Not to expensive and very well made which is probably why everybody is using them these days.

Attenuator, probably the 3"x3", 60 step, Shallco monster.

[nattonrice]

Two channels with some form of stepped attenuation (integrating Ti's delta would be pure win but a dact would be fine).

I like the idea of 4 unbalanced inputs and 2 balanced inputs using the vampire's that spritzer suggests above.

If the board has "KGITSOJC" silk screened on it then that would also be win

[Craig Sawyers]

This is another wild and crazy project - love it, and count me in!

I agree with the general format that others have come up with - predominantly RCA, with some limited balanced. No interest in Vinyl any more, for me main sources are CD, FM radio and DVD - none of which are balanced. Balanced out would be very useful for inputting to KG balanced headphone amps.

Eichmann (ETI) Phonopods are gaining a bit of a reputation, and may be an alternative to Vampire. WBT are expensive for what they are.

[digger945]

So after a successful T2 project, it is time to tackle something i've been playing with

on and off for quite some time. The KGITSOJC

The Kevin Gilmore in the style of John Curl preamp.

Now if you want to read over 700 pages of babble and bullshit, you can go over to diyaudio

and read the two threads.

I have.... and you're right.

Before i start laying out the board, i kind of need to know what people really

want.

What do you have in mind to switch between sources?

[atothex]

Awww shit. I want one of these already.

[spritzer]

Eichmann (ETI) Phonopods are gaining a bit of a reputation, and may be an alternative to Vampire. WBT are expensive for what they are.

What we really need are PCB mount units which basically leaves the Vampire units and the Cardas GRFA. Now if you want to P2P wire it then the sky is really the limit. I've always been quite fond of the WBT silvers but then I'm nuts.

What do you have in mind to switch between sources?

Not sure but something by Seiden, Elma or Shallco would work for me. I'd also like to switch the ground with the signal to interferance at bay from cheaper sources such as TV's etc. This means a lot of wires but locating the switch and volume control close to the input would minimize it. Since the Blowtorch is the inspiration for all of this, then I particularly don't like the layout they chose.

Speaking of the Blowtorch, it appears they used the P65 TKD stepped attenuators and a Seiden input selector. The TKD's aren't available in 4 gang but their range of 2 gang versions is pretty impressive. Horribly expensive but impressive with one version reaching 115kYen for a 2-gang unit. Now a DACT would suffice for most users or a relay based unit (both could be remote controlled with ease) but a quad RK50 or a large stepper works best for the nutcases amongst us.

[kevin gilmore]

I really like the idea of switching all 3 wires of a balanced, and 2 wires of an unbalanced input.

More relays. In fact an extra input relay that loads the input with something like 10k might be

a good idea too.

There is a rumor out there that the RK50 is soon to be discontinued.

In which case maybe the P&G might be the next best thing. Last i checked those were

upwards of $700... Or the relay step attenuator.

[justin]

I really like the idea of switching all 3 wires of a balanced, and 2 wires of an unbalanced input.

More relays. In fact an extra input relay that loads the input with something like 10k might be

a good idea too.

There is a rumor out there that the RK50 is soon to be discontinued.

In which case maybe the P&G might be the next best thing. Last i checked those were

upwards of $700... Or the relay step attenuator.

"3rd" wire of a balanced input should be connected directly to the enclosure anyway and to nothing else

[kevin gilmore]

that's the 4th wire, and very few people actually ever wire that.

Although mikhail was famous for tying pin 1 and pin 4 to chassis ground

which caused all sorts of trouble in his balanced creations.

Now most of you probably do not know how the krell preamps actually work.

The volume knob is totally faked. It is tied only to a stepper motor which is monitored

for current spikes as you rotate the knob. So the cpu counts the pulses and programs

a relay controlled attenuator. The remote control makes the stepper motor move which

moves the knob which the cpu counts pulses and programs the attenuator.

I could do the same thing with a motorized RK27. But instead of using the pot to actually

control the audio, the pot goes to the cpu which measures the single analog voltage and

then drives the relay controlled step attenuator. Similar to what justin does in the pico

except with relays instead of cmos switches.

Edited April 4, 2010 by kevin gilmore

[Pars]

You should do a vertical board for the back panel to save chassis footprint, with the Vampire PCB2F/S for single ended and something like the Neutrik NC3MBV-B for the balanced and equivalent female w/o lock tab. You could put the relays right on the board, maybe do 4 balanced/SE inputs, and balanced/SE monitor and pre outs. Muting relays as well.

Edited April 4, 2010 by Pars

[spritzer]

It would be a shame to see the RK50 go but it is a bit of dinosaur. Now of Khozmo have ever shipped anything then their large attenuator would be an option too.

I'm all for a full input/output board but I'd probably never use it. Might want to add invert phase too on top of the mute etc. features.

[kevin gilmore]

Invert phase, sure why not, the blowtorch had it.

Just 2 more relays... Maybe we can get to 50 pieces per board and get

a price break.

[n_maher]

I Last i checked those were

upwards of $700... Or the relay step attenuator.

Really? I'll check with my source but I swear they were under $500 last I checked (2ch going for $250) especially if bought in quantity. I'm not "in" for this project but I'll help source parts if I can.

[luvdunhill]

I've been down this road myself. I've learned a lot with this project, and can provide a few comments and observations, that might be helpful (probably not though).

First, I'm assuming you're using the Hitachi devices. The problem with the Renesas devices are poorer compliments compared to the original Hitachi devices. How do I know? Well, I had 1500 of them and about 3% of those are suitable for the Blowtorch clone. Yes, this was the (only) up side benefit of measuring all those devices for the DynaFET. In a somewhat related complaint, I had to use a distortion analyzer to dial in this circuit, in order to make up for the fact these devices aren't great compliments.

Second, I used high Idss input devices, which are almost impossible to source. Once you can get a hold of the new production Linear Systems devices, this will help this.

Third, I wish the circuit could swing more volts... that's my number one complaint. As you know, the limitation is the input devices, and a cross between Curl and Borbely (cascode JFETs) would be fantastic modification and would be my number one recommendation... yeah, I'd love to see this, please?!

Fourth, relays suck for attenuators. I'd recommend using CMOS switches or reed relays, or just drive the mechanical switch directly.

Fifth, the secret sauce here is the servo (I'm sure yours is better than mine) and keeping the thing thermally stable. To this end, a way to couple the devices together would be way cool and in my opinion, an important thing to consider in the layout. Perhaps like this:

Sixth, I think it would be cool to do a limited edition run of boards made from Teflon or Arlon, ala Borbely... ya, that would be sweet. Also, could you include the 0.2" pitch needed for Caddock MK132? In my circuit, these are by far the best sounding resistors.

Seventh, please include a Vendetta phono stage as well. k thx bye.

[luvdunhill]

[kevin gilmore]

I have studied the JC design for many moons. I have synthesized it over and over

and tweeked it over and over. Which is how i improved on the original.

Nope, not using the renasys devices

Yes i certainly am going to use the lsk389 and lsj109

My version swings 8 volts peak to peak (unbalanced) from +/-20 volt power supplies.

(may go to 24 volt supplies)

At 8 volts peak to peak the second harmonic is down 81db.

The 3rd harmonic is below the 120db resolution of my instrument.

At 2 volts peak to peak (unbalanced) the thd is about .005%

I know how to get the second harmonic down another 4 or 5 db.

But as many already know, its the sound that i'm after, not the numbers.

The problem with cmos switches is well... they are cmos switches. I know

that levinson uses cmos switches in their silly priced preamp, on an arlon board.

But i think that gold cross point relays sound better. I was planning on using

the omron small signal relays. But sealed reed relays would be better, but

are also much much bigger. Will see what i can get to fit.

Yes i think i have a pretty good servo. And its not in the audio path anymore.

Although you might argue that the common mode servo in JC's original is not

in the audio path because it is DC, except when you go balanced input.

I've pulled out my schematics for my phono stage from 30 years ago. I'm pretty

sure i can source equivalent parts for everything, so at some point there will

be a phono stage. OH look, there is the lsk389 and lsj109 again...

An exact copy of the vendetta phono preamp should be a snap.

But in a seperate box with a seperate power supply.

.2 pitch OK, glad you said something now because i'm going to start laying

out the board tomorrow.

Can't wait to see the price on teflon or arlon circuit boards.

Just wait till you see how silly the power supply is going to be.

Edited April 5, 2010 by kevin gilmore

[luvdunhill]

cool. Cannot wait! Oh, and Mr Hansen seems to always be raising the bar as well

[deepak]

The KX-R attenuator does sound really cool. Incredible sounding preamp too.

[Craig Sawyers]

There is a rumor out there that the RK50 is soon to be discontinued.

It has disappeared from Alp's website product line up for sure.

In which case maybe the P&G might be the next best thing. Last i checked those were

upwards of $700... Or the relay step attenuator.

I've had a recent quote from P&G when I was toying with the idea of using one in the T2. Their prices for the RF15 in one off quantity in UK pounds was:

Two gang RF15/2/D/A,

[Craig Sawyers]

a cross between Curl and Borbely (cascode JFETs) would be fantastic modification and would be my number one recommendation... yeah, I'd love to see this, please?!

I've corresponded extensively with Erno Borbely in the last couple of years. He is seeking to retire during 2010, and shut up shop. He is trying to find someone to take over his designs, and either supply kits or built product for a suitable commercial deal and ongoing support. I've tried in the UK with Russ Andrews and Martin Colloms, so far with no luck. It would be a shame if his designs and design philosophy went the way of all dust.

Any takers from the Industry?

[spritzer]

Here is a larger picture of the CTC unit:

The Ayre is something else though with a rather advanced attenuator. Something to look into perhaps...

I've had a recent quote from P&G when I was toying with the idea of using one in the T2. Their prices for the RF15 in one off quantity in UK pounds was:

Two gang RF15/2/D/A,

[Craig Sawyers]

Not a bad price at all! Can they supply anything higher then 10K though?

Not as standard - 1k, 5k and 10k only. I've heard rumours on the web of higher values, and I've reasonable confidence that they will do custom values - but I have no idea at what volumes or unit price they get interested. If we have a value we're particularly interested in, I can certainly ask the question.

I visited them in Wales around 20 years ago, and at that stage they were surprisingly cottage industry like. I think they are much more professional now, and part of a much larger parent.

As I recall, to make the track they basically silk screen print resistive paste onto a polished glass plate and fire it. They then mould the plastic support plate with contact pins over the track. It is quite a while - so I can't remember any further details, and there must have been quite a few.