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Old 27th January 2013, 05:57 PM   #21
diyAudio Member
Join Date: Jan 2013
Default Preamp

Hi Marvinkite,

Wow, this guitar looks like a lot of fine handwork. Great!!!

I attach the (not actual) schematic of the Smart DI. I think you can use the preamp part as well as the balance part of it without problems. The LT1014 is an old device with a limited bandwidth. The most modern op amps have a much higher slew rates but I found out that they often tend to "ring" (I donīt know the correct word in English...) when they have to drive capacitive loads like a long xlr cable. A lower bandwidth op amp have usually not this problem. About a poti: I think, just a poti instead of the 2 BF245 would do the job.

In the old di I used in the output section smd ceramic capacitors cause they were faster to handle and keep the cost down. After some rehearsals with different capacitors we changed to film capacitors in the output section. These parts are bigger compared to the ceramic and need to be placed by hand but the sound was better compared to ceramic cīs. The input capacitors ars still in smd. We had not heard any differences in the sound compared to the film capacitors. Maybee you might hear a difference. The 24 Ohm resistors are already removed. I placed it before in order to prevent the op amps outputs against "burn" but the 1000 uF capacitors in the circuit will make a soft charging in every way. About the input section: A very high impedance input will help to reduce a coloration of the sound. So, the 27k for the attenuation is not necessary in your case, I guess.

However, just contact me if you have any questions about the schematic. It might looks a bit confused and could need a "clean up". Sorry in advance if I will not answer very fast as Iīm on a trade show next week and I donīt know if I have internet access in the hotel...

All the best!
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File Type: bmp DI.BMP (189.5 KB, 81 views)
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Old 28th January 2013, 05:26 AM   #22
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Join Date: Mar 2012
Default WOW!

Hey thanks for the compliments! I checked out your website and your product line...AWESOME! I asume you're into music if not a musician. I remember my soundman days, when direct boxes were just starting to come out. Thanks for sending me the circuit. It looks like a LT1014 is a quad. I also see you have some great filtering of the phantom power coming in from the XLR. Whew! Lots of smoothing. Do you recommend it? I'm planning on only utilizing a balanced driver/gain stage in one op amp. I have a design I made with a shoeps front end, a modded linkwitz electret capsule, and a LME 49724 balanced line driver. I don't even have a volume control (yet) and have questions about my circuit. I'm not sure if the way I have the phantom power coming in will work. Take a look. I have question marks on values that I don't know. I don't even know if this combo will work. My thing is more the guitar designing.
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File Type: pdf Phantom powered sch-2.pdf (11.2 KB, 75 views)
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Old 31st January 2013, 09:50 AM   #23
teleman is offline teleman  Norway
diyAudio Member
Join Date: Jul 2008
Marwinkite, Have you tried your circuit? You could perhaps assemble/build it on a bread/perfo board & test it? You don't need a trillion ic mic buffer circuit & I would keep it as simple as possible if I were you! Another alternative would be the popular, well tried & tested BB INA217 based balanced mic circuit. Please refer to the circuit on page 7 in the attached PDF doc.This is supposed to be excellent & is used in numerous professional mic pres!

You may have to change to a higher value rev. log volume control to reduce the original gain of 60dB, which you certinly don't need for your guitar! You may also need to increase the lower -3dB point to around 100Hz, since anything under this freq;would result in unconrollable LF feedback! You can also omit the optional DC output loop ic OPA137!
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File Type: pdf INA217.pdf (679.6 KB, 62 views)

Last edited by teleman; 31st January 2013 at 09:57 AM.
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Old 31st January 2013, 10:22 PM   #24
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Join Date: Mar 2012

Hey Teleman, I've already looked at this circuit. Nice for matching up to a loooong mic cable, but not so good for a very short run of wire (about 300mm) from the bridge to the side of the guitar. It also has a single ended output, which would NOT be suitable for a built-in preamp running from the guitar to a mixer through a balanced XLR mic cord. It would turn into a great antenna for God knows what. Also, I would have to come up with a suitable 15 volt supply through voltage dividing the phantom power, or adding voltage regulators plus and minus...all complicating - not simplifying the circuitry. Soooo, I am looking to simplify the circuit (don't need anything beside a voltage divider for the plus/minus power (plus filtering), and a way to bias the linkwitz modded capsule, really...although as pointed out, a volume (or gain) control might be essential. It's just not easy with balanced signal lines. Everything has to be 'ganged', and so must be well matched to maintain signal integrity. I DO believe in balanced signal lines (twice the signal, half the noise) and need to maintain that as a requirement. I really appreciate your help and knowledge.
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Old 1st February 2013, 06:55 AM   #25
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Join Date: Jan 2013
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Hi Marvinkite,

Yes, I tried smaller capacitors for the + / - 6 Volt of the LT1014 but I found out that high level signals with a low crest factor (like a keyboard with "standing" tone) increased the current consumption of the LT1014 what followed in a bit reduced supply voltages for the LT1014. This reduced the dynamic. For myself it sounded more clear with bigger capacitors but Iīm sure you donīt might need these big capacitors cause an acoustic guitar have a much much higher crest factor compared to a keyboard what will reduce the discharge a lot. However, just try out different values by yourself.

About the input impedance: I tried before to use 5 M-Ohm resistors in the input section but the internal input offset current of the LT1014 followed in a offset voltage after the first stage. So, I reduced the input resistor to 1,5 M-Ohm. This is just in the range what other good instrument diīs usually have. I wished to increase the input resistor value but this was not possible with the LT1014. However there might be other op amps on the market with a lower input offset current what will allow a higher input resistor value.

All the best!
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Old 4th February 2013, 12:35 PM   #26
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Join Date: Aug 2012
Location: Markneukirchen, Germany
Hey guys! Referring to OP, here's an article by Michael Kellett, which I've found extremely useful for understanding charge amps. Although the application in the article is instrumentation, specifically piezo accelerometers, the basic idea is explained really well and that circuit can be tweaked to work for any piezo, even for audio application (I've tried it!)
Hope it will help someone who comes across the thread!

@jabeavers, if I can suggest something, I would try using lower value pots in the EQ section, try using 5K or 10K pots, as per my experience it helps to reduce the noise floor of the filter.


Last edited by WindMarp; 4th February 2013 at 12:37 PM.
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