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A Stereo Instrument Preamp/Headphone Amp (formerly "a JISBOS-with-gain Headamp") - Page 7

post #91 of 109
1/16/10 at 2:21am
Quote:
Originally Posted by Zaubertuba View Post
WOW! A thousand thank-you's, kind sir! *Bows gregariously in your general direction.*
You're quite welcome.

Quote:
How did you draw that? It looks gorgeous!
Thank you.

I used a program called Xara, which is a vector-based graphics program similar to CorelDRAW.

I never liked the schematics produced by schematic capture programs and wanted publication quality schematics instead. So I first started doing them in AutoCAD many years ago. In the late 90's I ported the symbols I'd created in AutoCAD into Xara and have been doing them with that ever since.

se
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post #92 of 109
1/16/10 at 3:49pm
Thread Starter 
Quote:
Originally Posted by luvdunhill View Post
I like. The only thing I wouldn't want from that configuration is gain.
Let's think about this a second. I was planning on adding a little gain to the JISBOS boards. What are the postives/negatives of actually using step-up transformers (Steve I see those Jensen's-in-a-box are still on eBay--guy hasn't sold them yet), and keeping the JISBOS at unity gain?

I figure the JISBOS boards could certainly handle it--they were designed as an line output stage for the SOHA amp, anyway.
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post #93 of 109
1/16/10 at 4:47pm
Quote:
Originally Posted by Zaubertuba View Post
Let's think about this a second. I was planning on adding a little gain to the JISBOS boards. What are the postives/negatives of actually using step-up transformers (Steve I see those Jensen's-in-a-box are still on eBay--guy hasn't sold them yet), and keeping the JISBOS at unity gain?
The positives are gain with very low noise, very low distortion, no power supply issues, etc.

The negatives are that you won't have quite the flexibility of sources that you'd have otherwise. By that I mean that the peak outputs should be a few volts or less, source impedance should be a few hundred ohms or less.

By the way, before taking this any further, what sort of flexibility do you have with regard to the value of the faders you're going to use for volume control?

se
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post #94 of 109
1/17/10 at 12:35am
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
Why's that?

It's the same configuration I'm using for this, except I'm using step-up trannies instead of the 1:1's I drew for Zaubertuba's circuit
I'd like something that would work for any JFET input balanced amplifier. Also, assuming that this includes designs that aren't necessarily unity-gain stable, this means there will possibly be a lot of gain in the amplifier already. Thus, I'd like to choose a transformer that doesn't offer any appreciable gain. So, this is what I'm on the lookout for.
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post #95 of 109
1/17/10 at 12:51am
Quote:
Originally Posted by luvdunhill View Post
I'd like something that would work for any JFET input balanced amplifier. Also, assuming that this includes designs that aren't necessarily unity-gain stable, this means there will possibly be a lot of gain in the amplifier already. Thus, I'd like to choose a transformer that doesn't offer any appreciable gain. So, this is what I'm on the lookout for.
What exactly is the puzzle that you're wanting the transformers to be a piece of?

se
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post #96 of 109
1/17/10 at 1:10am
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
What exactly is the puzzle that you're wanting the transformers to be a piece of?

se
single ended to balanced conversion and other benefits that input transformers offer, without loosing half my gain compared to balanced operation (so, phase splitting, assuming four identical gain blocks)
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post #97 of 109
1/17/10 at 2:08am
Quote:
Originally Posted by luvdunhill View Post
single ended to balanced conversion and other benefits that input transformers offer, without loosing half my gain compared to balanced operation (so, phase splitting, assuming four identical gain blocks)
Ah, gotcha.

se
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post #98 of 109
1/17/10 at 2:10am
Thread Starter 
Quote:
Originally Posted by Koyaan I. Sqatsi View Post
The positives are gain with very low noise, very low distortion, no power supply issues, etc.
Sounding better all the time...

Quote:
The negatives are that you won't have quite the flexibility of sources that you'd have otherwise. By that I mean that the peak outputs should be a few volts or less, source impedance should be a few hundred ohms or less.
Well, as long as I don't have to ramp back my 1212m *too* much, I think it could handle it. Most consumer products are running at around 150 ohms at the output anyway, right? I think most portable players are running even lower than that, so my Wife's Fuze would probably work fine as well.

Quote:
By the way, before taking this any further, what sort of flexibility do you have with regard to the value of the faders you're going to use for volume control?
The faders just came today, actually. They're Penny & Giles model number PGF4222. The guy who sold me them told me they're comparable to the PFG3000 series. They're 5k, log taper.

There is a note on the PFG3222 spec. sheet that says, "The ratio of fader resistance to wiper load should be 100:1 or higher."
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post #99 of 109
1/17/10 at 2:34am
Quote:
Originally Posted by Zaubertuba View Post
Well, as long as I don't have to ramp back my 1212m *too* much, I think it could handle it. Most consumer products are running at around 150 ohms at the output anyway, right? I think most portable players are running even lower than that, so my Wife's Fuze would probably work fine as well.
Seeing as your faders are 5k's, then you'll want to use 1:1's, which means they'll be able to handle considerably higher input levels and source impedances. Then use the JISBOS for your gain instead.

Quote:
The faders just came today, actually. They're Penny & Giles model number PGF4222. The guy who sold me them told me they're comparable to the PFG3000 series. They're 5k, log taper.
Sweeeet!

Quote:
There is a note on the PFG3222 spec. sheet that says, "The ratio of fader resistance to wiper load should be 100:1 or higher."
Not a problem. If you build the JIBOS as per the schematic, the input impedance will be around 1M.

se
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post #100 of 109
1/22/10 at 4:41am
So a few pages back I posted a mini review of the JISBOS boards I was powering with an unregulated PSU. Recently I've been using them as the single ended output stage for my Opus DAC and until I get another amp put together, I'm running a pair of HF-2s straight off the output using the foobar volume control. I also upgraded to a 7812/7912 +/- 12V regulated PSU.

It sounds fantastic. There are tons of details I haven't noticed before, the bass is very very deep and detailed, and the soundstage seems bigger than out of the Bijou (at least with this pair of headphones, I also don't have the Bijou here to compare directly). I think they're a great match with the HF-2s and can make a very decent amp. I think you'll be very satisfied when you put your whole project together.
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post #101 of 109
3/29/10 at 1:13am
Thread Starter 

Yet another twist....

Sorry I've been gone so long. I've had a little "detour:"

I haven't played bass in 20 years and a friend lent me an instrument a few months ago--one of these "tapping" instruments with wayyy too many strings--but I've been able to play with no pain for the first time in a long while. Actually played a jazz gig just a few weeks ago.

Anyway, this does relate to the topic...I'll get to the point: Since it's transformer-coupled and I'm using reasonably decent components, I was thinking of adapting this design to also serve as a practice amp. Right now I have to plug into my computer and route through my E-Mu's patchmix software--works, but it's fiddly. I'd also like to use it as an instrument preamp for gigs.

The pickups on the instrument I'm looking at getting (Stick Enterprises NS/Stick) are active EMG's, output impedance is 10k-ohms. Would I need to add instrument-level input stages (there'd be two--the NS has an output each for the high and low strings)?

Some people over on the Stickist forum have suggested a Mosfet Preamp circuit:

Stickist.com - View topic - DIY Stick Preamp Prototype Project - Part 1

I'd probably be sending the guitar-side of the instrument through an effects unit, so that'd be line-level, but I'd like to keep the option to run the bass side directly into the preamp.

Thoughts?
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post #102 of 109
3/29/10 at 1:18am
Thread Starter 
Quote:
Originally Posted by mugdecoffee View Post
So a few pages back I posted a mini review of the JISBOS boards I was powering with an unregulated PSU. Recently I've been using them as the single ended output stage for my Opus DAC and until I get another amp put together, I'm running a pair of HF-2s straight off the output using the foobar volume control. I also upgraded to a 7812/7912 +/- 12V regulated PSU.

It sounds fantastic. There are tons of details I haven't noticed before, the bass is very very deep and detailed, and the soundstage seems bigger than out of the Bijou (at least with this pair of headphones, I also don't have the Bijou here to compare directly). I think they're a great match with the HF-2s and can make a very decent amp. I think you'll be very satisfied when you put your whole project together.
This is very encouraging. I did finally order and get my kits from GJA. Now to get the power supply, transformers, connectors....and any new stuff I may need to add the "instrument pre-amp" functionality.
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post #103 of 109
9/20/10 at 11:41pm
Thread Starter 

 

O.K., this went in a completely different direction then I thought it would...just got done reworking my concept into much more of a dual-channel instrument mixer/preamp with great headphone out capability.  
 
The (re-worked) block diagram (larger version):
duetblockdiagramb.jpg
 
 
 
Changes to the Design Brief:
 
-2 discrete instrument input channels with EQ and an Aux/FX send.
 
-2 Line-in inputs for FX return or other input sources.
 
-Stereo bus input stage architecture for maximum flexibility.
 
-Switchable to balanced direct inputs for use as a balanced line-driver/dedicated headphone amplifier.
 
-Neutrik Combo output jacks to drive XLR-terminated balanced headphones or other balanced line equipment.  1/4" outputs can be used for the same purpose, or, when the mode switch is engaged, can provide two separately buffered, single-ended stereo outputs for driving two pairs of headphones.
 
With this design, you could run a stereo instrument into a stereo P.A. with complete balance and level control, run a stage amp/monitor and send a separate signal to FOH.  Mix in other instruments, two completely discrete (and separately pannable) effects loops....there's almost too many applications to list.
 
It's a bit of a "trick" using the negative phase signal from the transformers to provide two separate single-ended signals.  As mentioned earlier in the thread, the cost is that the second stereo signal is out of phase with the first.  Obviously I have to work some sort of phase-reversal in order for any other on-stage applications to work.
 
One dilemna I have is what exacty to use for the instrument preamp/EQ.  I posted a query on TalkBass asking this question (I know, I'm such a forum-whore!  ) , but if there's any diy guitarists/bassists around here who'd like to chime in I'd be glad to hear from you!
 
Some current options:
 
PAIA tubehead preamp:
 
 
..or a couple Rod Elliot designs:
 
 
 
 
I am slightly concerned about retaining a quality input signal, what with all the routing on the instrument input stage.  I guess that's why I included a source switch that couples just the balanced inputs directly to the transformers.  If there's a more elegant way to do this I'd be glad to hear of it.

Edited by Zaubertuba - 9/21/10 at 6:54am
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post #104 of 109
9/23/10 at 4:52pm
Thread Starter 

I'm still wondering about this design.  Though it accomplishes nearly everything I think is important to me, I'll still have to do *something* to invert the phase of the second stereo output channel if I want to utilize it in a live performance setting. I don't really want to introduce any more integrated circuitry after the input selector--the whole second half of this thing is more of a simple Class-A approach. Maybe I need to look at utilizing the transformers in a different way... Hmmm....

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post #105 of 109
10/11/10 at 10:33pm
Thread Starter 

I found this great Nelson Pass article: http://www.passdiy.com/pdf/diyopamp.pdf

 
I'm thinking of doing a bread board mockup of this ESP parametric:
 
p28_fig1.gif
 
 
Only substituting using a discrete jfet opamp (along the lines of fig. 16 in the article) in place of the input buffer, and perhaps as the make-up amp after the parametric as well.  A few questions:
 
1.) I'd like to keep to 15v rails on this project (that's the recommended voltage for the JISBOS buffers, and several other circuits I'm considering).  Mr. Pass mentions in the article that reducing the rails (with the appropriate bias corrections) from the 32v tested in his examples has a negligible effect.  However, since the article's fairly old and I need find a substitute jfet anyway, does anybody have a recommendation for a preferred jfet that will spec out nicely at the lower rail voltage?
 
2.) (Angels-on-a-pin question wink.gif ) ...Just how large a batch of jfets do I need to test to be able to get one (or two) reasonably matched pairs?

Edited by Zaubertuba - 10/11/10 at 10:36pm
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