Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

PA Systems A forum for discussion of all parts of a sound reinforcement or DJ system: loudspeakers, mixers (desks) etc.

Solid-State Balanced Microphone Preamplifier
Solid-State Balanced Microphone Preamplifier
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th June 2019, 08:42 PM   #91
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
If the color control works, this maybe good for the first production version. A single pole switch and series resistor with the pot is needed to enable and limit the action. Bipolar electrolytic or multi-layer SMD ceramic would keep the board polarity neutral. Another thought is to bring the connections out to pin header strips. Include the emitters and collectors for further experimentation. The controls could then be mounted on a mezzanine using PCB pots and switches. This would simplify the build for other DIYers.
  Reply With Quote
Old 18th June 2019, 07:21 AM   #92
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
I'll try to test the distortion control in the next few days. At the moment I've got a couple of control boards for an old magnet power supply at work on my bench which take priority, but I should be done fixing those in the next day or two.

I like the latest iteration of the distortion control. It's definitely a good idea, and I certainly would prefer to add color with analog circuitry rather than software. More subtle and less invasive is good as well. In addition, it is pretty simple and won't require drastic PCB changes.
  Reply With Quote
Old 20th June 2019, 01:02 AM   #93
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
There is a further option, bridging the collectors of the second stage. This creates a different profile, promoting odd over even distortion (according to LTspice). This, or version 2 could be selected with a on-off-on switch, since only one end of the bridge is being shifted.
Attached Files
File Type: asc H713_FET_2SK3557_color_3.asc (6.2 KB, 5 views)
  Reply With Quote
Old 20th June 2019, 07:45 AM   #94
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
Hopefully in the next few days I'll get the other stuff off my bench and have a chance to play with the distortion control- it could certainly make this preamp more interesting.

I did have a bit of time during my lunch break today to play with my simulation file, and with a few resistor tweaks it should be possible to get the gain down to considerably less than 8dB. How the stability will be with that much global negative feedback, we will have to see. I may turn a couple of the rev. 1 boards into a line stage to see how it works- I think it could perform extremely well. Distortion looks good, the only unknown may be stability and how well the gain adjustment works, though for a line stage noise is less of an issue and RV1 could be replaced with a fixed resistor and a balanced attenuator added to the front end.

Since the gain stage is proving to be far more versatile than I originally expected, would there be any interest in a smaller PCB without all of the input pad and phantom power clutter? Something along the lines of a balanced amplifier module that could also be configured with a fixed amount of gain rather than variable.

With the relatively beefy output transistors, it might even be possible to drive high-impedance (300 or 600 ohm) headphones directly off the output.

As it stands, I see no reason to change the BD139s. Though they are large, they work well. I've tried TO-92 packages in that place, and while it works fine, it is very easy to kill them. The BD139s are almost bulletproof in this application.
  Reply With Quote
Old 25th June 2019, 08:35 AM   #95
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
Experimentation with P and N versions spliced for common mode distortion cancellation produced some interesting results. A general purpose preamp seems viable with the capability to drive headphones. Two versions are attached. The desktop version delivers class A up to 1V RMS into 32R which is near live band levels for a lot of phones. Class B is then available for brain mincing. Distortion is indicated at below 0.001% at 20Khz, with still reasonable figures up to 500KHz. At higher impedance loads, the distortion plummets.

Open loop bandwidth is beyond 100KHz and feedback is around 20dB. The input bases are self biasing , no resistors needed. The design is self protecting and to a degree also protects the headphones. Gain can be switched in if required. The number of devices can be minimized with dual transistors. A practical version could be very compact.

A lower quiescent current version could be used as a portable device.
Attached Files
File Type: asc H713_BJT_NP_headphone_desktop_1a.asc (12.5 KB, 7 views)
File Type: asc H713_BJT_NP_headphone_portable.asc (13.1 KB, 3 views)
  Reply With Quote
Old 2nd July 2019, 09:21 AM   #96
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
An idea for a very simple one control compressor. Two additional transistors drive a LED which is coupled to a cadmium sulfide photocell. This forms the gain control which can be switched in, in parallel with the gain pot. There is a quirk in this approach. The photocell attack and release times are asymmetrical. In this case, the attack time is slower (LED getting dimmer). However, photocells have hysteresis that evens out changes which might make the configuration usable. A follow up would be to use a side chain based on this approach and implement an attenuator in a manner similar to the LA-2A. The LED would then be reversed and the cathode connected to the negative rail with the anode driven by the emitters.
Attached Files
File Type: asc H713_FET_simple_comp.asc (7.4 KB, 6 views)

Last edited by spladski; 2nd July 2019 at 09:43 AM. Reason: Anode and cathode swapped
  Reply With Quote
Old 5th July 2019, 11:24 AM   #97
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
From the LA-2A mold, a side chain compressor. Some additions are present. HF boost or cut for keying, with turnover via selection of caps. A dual-gang control is needed for HF cut. The zener for the compressor LEDs is chosen so that with no input, the LEDs are just off. Additional diodes could be switched in series with the zener, so that compressor action is prevented until a certain level is reached. The slope control adjusts the brightness of the LEDs.
The side chain is parallel with the mic amp sharing the same input. The compressor LEDs and zener can be adjusted so that more LEDs can be added. This is to allow a variety of LDRs which may need to be paralleled due to high impedance of some photo cells.
A LED headroom meter is thrown in. The LEDs are not entirely hard switch on, but should be usable. The good thing is that no currents are pumped into ground.
Attached Images
File Type: png H713_side_chain_comp.PNG (49.7 KB, 63 views)
Attached Files
File Type: asc H713_FET_side_chain_comp.asc (14.6 KB, 3 views)

Last edited by spladski; 5th July 2019 at 11:29 AM. Reason: typo
  Reply With Quote
Old 5th July 2019, 12:13 PM   #98
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
The last step for a channel strip is an eq section. This could be passive, or active based on the differential amp module. Ideas anyone?
  Reply With Quote
Old 6th July 2019, 10:13 PM   #99
spladski is offline spladski  United Kingdom
diyAudio Member
 
Join Date: May 2012
This is what we have so far. Leaning towards passive eq due to amount of R&D effort and ease of build. Unless some one comes up with a brilliant plan. As far as the modules go, fixed configurations may be useful for novices. What these design exercises have shown, is that an SMD uncommitted PCB with just transistor pads is useful because it can be populated with P or N, with a module on both sides. Therefore a two-sided module could be:
1. Dual channel
2. One channel + side chain
3. P and N for the distortion cancelling version
One module covers multiple functions because the transistor footprints don't change for P or N. The module could be edge mounted, or like a big DIL.
Attached Images
File Type: png H713_strip_1a.PNG (25.5 KB, 46 views)
  Reply With Quote
Old 8th July 2019, 07:11 AM   #100
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
Color controls may or may not get implemented into the desk- we shall see. I still have to find time to play around with that circuit, but at the moment I've got family in town, and that is consuming a lot of the time that would normally get dedicated to projects- so progress is slow but steady.

I'm definitely going to make a hybrid IC around this, since I think that's about the best way to utilize a versatile circuit. I have yet to test the FET version, mostly because I forgot to add the 2SK3557 JFETs to my cart last time I ordered from mouser.

It will be a full channel strip- ultimately. My next step with this project is going to be to build it into a line stage (great use for some of the remaining rev. 1 boards with the miswired pad switch) and a 4 channel 1u mic preamp. I think before I put much more energy into this design, I want to see how it performs in the real world. Simulations, bench testing and time on the AP box is all well and good, but if it sounds like dog dung, what exactly is the point? real-world testing of a line-stage module looked great. Distortion generally stayed below .009% for all reasonable output levels. I consider that to be respectable. The line stage won't have an input transformer, but it will have an output transformer.

I think I've tracked down a usable chassis for the 4 channel 1u mic pre, and I should have something laying in storage for the line stage. Progress is being made, ever so slowly. Am I the only one who never has fewer than three or four projects going at once?

Depending on if I can learn how to do so, I may try to design my own input and output transformers for this thing as well- that's a little down the road.

Also, my order of 10 Rev. 2 boards came in today, and they look reasonable. Now I've got to actually build something with them.

H713
  Reply With Quote

Reply


Solid-State Balanced Microphone PreamplifierHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Balanced Microphone Preamplifier suneetha Instruments and Amps 11 9th October 2014 08:15 PM
solid state preamplifier ecodoppler Solid State 9 7th October 2005 02:21 PM
Tube preamplifier & Solid State power amplifier for guitar Dusk Instruments and Amps 2 13th October 2004 10:48 AM
Balanced Preamp with discrete solid state volume control. Dave Solid State 6 20th August 2004 10:17 PM
:: minimalist solid state preamplifier :: dhengkoel Solid State 10 26th August 2002 12:39 PM


New To Site? Need Help?

All times are GMT. The time now is 05:44 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki