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Double Bass How to make a piezo preamp

#21
I made one of these preamps today. Couldn't source a 2N5486 yet but the local store had some 2N5485's. Initially I'll put one in a friend's EUB so I made it on a scrap of perf-board that I can stick on the back of its battery holder.

This is one of those rare occasions where I make a circuit that works first time!

First up, I tested it with a piezo into my Eden WT330. It worked fine, sounded exactly the same as without the preamp - clean, no noise, no noticeable gain. (The Eden obviously has a fairly high input impedance that works OK for piezos)

Next, I tried it into my '63 Goldentone Bassmaster tube amp. This time there was a huge improvement. Without the preamp the piezo was thin and nasty; with preamp there was a big fat sound. Obviously this amp had been loading down the piezo and the buffering has restored the full frequency response.

Thanks fdeck, this little circuit is great! I'll be making more of them.
 

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#22
Tone Ranger,
You win the prize for esoteric bass gear. I thought I knew all the different amplifiers out there made from the 60's, 70's and 80's, including Jordan, Bruce, and Magnetone. I even owned a Casino 2x15 (Customs foray into the Solid State arena). What's a Golden Tone Bassmaster?


Ric
 
#23
Ric Vice said:
Tone Ranger,
You win the prize for esoteric bass gear. I thought I knew all the different amplifiers out there made from the 60's, 70's and 80's, including Jordan, Bruce, and Magnetone. I even owned a Casino 2x15 (Customs foray into the Solid State arena). What's a Golden Tone Bassmaster?
Click to expand...
Well, they're not that esoteric if you live in Australia!
Goldentone was one of several good Aussie amp makers in the 50's & 60's (Moody, VASE, Vadis and Wasp were some others).
Mine is a sweet little 40 watt head that I play through an old Jensen 250w 18" speaker. Not enough power for gigging, but great for practice & recording. Also excellent as a guitar amp (think Fender Bassman - main difference being that I got this one for $150).
 
#24
ToneRanger said:
Well, they're not that esoteric if you live in Australia!
Goldentone was one of several good Aussie amp makers in the 50's & 60's (Moody, VASE, Vadis and Wasp were some others).
Mine is a sweet little 40 watt head that I play through an old Jensen 250w 18" speaker. Not enough power for gigging, but great for practice & recording. Also excellent as a guitar amp (think Fender Bassman - main difference being that I got this one for $150).
Click to expand...

Apologies to fDeck for derailing the thread, but ToneRanger, if you ever have the time to snap a picture of your Goldentone Bassmaster. I'd love to see it. This stuff always interests me. Unfortunately, I'm way to sloppy with a soldering iron to built this unit, but it sure is cool!

Ric
 
#25
ToneRanger,
Actually, there is no need to take those pictures unless you want to. I found one on the net. Here is a shot of that little beauty for everyone to google at. I really dig the string bass logo on the left side of the head, it's classic.
 
#27
ToneRanger said:
Yep, that's the one, although mine still has the original chicken-head knobs, gold fleck vinyl and gold grille cloth.
Click to expand...

Chicken head knobs and gold fleck vinyl with a gold grille cloth would make it a vintage Goldentone!!!! If you ever get a chance send some picts.

Ric

PS: fDeck I now return this thread to its regularly scheduled programming.
 
#28
fdeck said:
...for a volume control, I would just hang a 10k audio taper potentiometer on the output (after the capacitor).
Click to expand...

By 'hang on the output' do you mean connect the pot across the output and the ground and take the wiper as the feed, or with the pot in series with the output? (sorry it's been a REALY long while since I graduated as an electronics and computing major).

Thanks for the design - and great timing as I'm having impedance issues with my new Realist.

While I think about it, what would be the best way of linking two of these together to make a little mixer (for my realist & bassmax). Perhaps a balance pot - pre-amp outs to each end and the wiper as the mix output? What value might be best?

Thanks,
 
#29
Ric Vice said:
Chicken head knobs and gold fleck vinyl with a gold grille cloth would make it a vintage Goldentone!!!! If you ever get a chance send some picts.

Ric

PS: fDeck I now return this thread to its regularly scheduled programming.
Click to expand...
No problem, I enjoyed the digression, especially learning the new term "chicken head knobs."
 
#30
airbass said:
By 'hang on the output' do you mean connect the pot across the output and the ground and take the wiper as the feed, or with the pot in series with the output? (sorry it's been a REALY long while since I graduated as an electronics and computing major).

Thanks for the design - and great timing as I'm having impedance issues with my new Realist.

While I think about it, what would be the best way of linking two of these together to make a little mixer (for my realist & bassmax). Perhaps a balance pot - pre-amp outs to each end and the wiper as the mix output? What value might be best?

Thanks,
Click to expand...
Pot across output and ground, with wiper as the feed.

Now, about mixing, tell me how much of an actual gain difference you need to balance. One idea is to look up the classic circuit for mixing two pickups in a passive electric bass. I will search for the circuit too, and see if it can be adapted. The problem with any passive mixing arrangement is that you soon end up with a high impedance feed to the amp, such as you are trying to avoid in the first place.
 
#31
By the way, Toneranger, nice build. I see you socketed the JFET. Maybe we will get into JFET swapping like those electric bass guys do with their tubes.

I uploaded a new file, with a couple more treats. First is a two channel preamp with pan, volume, and a phase switch. Second is a single channel preamp that runs on a phantom power microphone cable, sans battery. It is a simplified version of a circuit that I designed for a multi-instrumentalist friend who was running three piezo pickups on stage, and was tired of managing all of the batteries.
 
#32
I'm just about to build your buffer design. Here in germany I have difficulties to get one of your suggested jfet types. I however managed to get a batch of BF245A. Are they ok, too? I would like to understand which parameters of the data sheet are qualifying i.e. the 2N5486 as great?
I found a similar design using the BF245 here:
[Invalid or Expired Link Removed]
I searched the net and learned a lot about biasing a jfet amplifier but found nothing about selecting a value for Rs in a source follower circuit. Could you please explain how to find the right value? Should I use 4k7 as in the link I posted or is the value uncritical? Also I would like to know how to select a value for the output capacitor!
I would like to understand which parameters of the data sheet are qualifying i.e. the 2N5486 as great?
Any help would be highly appreciated!
 
#33
astro said:
I'm just about to build your buffer design. Here in germany I have difficulties to get one of your suggested jfet types. I however managed to get a batch of BF245A. Are they ok, too? I would like to understand which parameters of the data sheet are qualifying i.e. the 2N5486 as great?
I found a similar design using the BF245 here:
[Invalid or Expired Link Removed]
I searched the net and learned a lot about biasing a jfet amplifier but found nothing about selecting a value for Rs in a source follower circuit. Could you please explain how to find the right value? Should I use 4k7 as in the link I posted or is the value uncritical? Also I would like to know how to select a value for the output capacitor!
I would like to understand which parameters of the data sheet are qualifying i.e. the 2N5486 as great?
Any help would be highly appreciated!
Click to expand...
I chose the 2N5486 by modeling the circuit in LTSpice (at Link Removed) and choosing a JFET for the widest dynamic range. However, both the JFET and the source resistor are uncritical. I found a model for the BF245A on the Web, added it to the library in LTSpice, modeled the circuit, and I think it will work just fine. Most importantly, when running on 9-V supply with ground referenced input, the circuit handles more than 2 V p-p input amplitude.

For the output capacitor, I chose C > 1/2/Pi/R/f

R = the desired output impedance, I chose 10k Ohms
f = lowest desired frequency, say 15 Hz

This gives C = 1 uF, and I had a pile of 10 uF capacitors in my parts box.

If you want to model the BF245A, here is the model:

.MODEL BF245A NJF (VTO=-2 BETA=0.9M LAMBDA=6m RD=2.21. + RS=1.99 IS=26.3F PB=1 FC=.5 CGS=2.12P CGD=2.52P)

Just add that line to the file STANDARD.JFT in the LTSpice directory.
 
#35
After experimenting a bit more I finaly got it but I don't fully understand the principle of the source follower jfet circuit.
Why is it that JFETs with higher Vgsoff as the 5486 or the BF245C provide a higher dynamic range?
For regular amplifiing curcuits with the output connected to the drain a low Vgsoff is necessary to archive a high dynymic range, right?
I'm confused.
 
#36
Here is how I understand it. When the input is centered around 0 V (the gate resistor to ground makes this happen), the output is centered around Vgsoff. If this is roughly 1/2 the power supply voltage, then you get the most dynamic range. This is in an ideal world. You can determine the dynamic range by modeling the circuit with an unreasonably high input amplitude like 5 Volts, and try different JFETs. Oh, and I will send you the simulation file, even though it sounds like you got everything working.

The opposite is the case if you are taking the output from the drain. Here, Vgsoff plus the voltage across the JFET, plus the voltage across the drain resistor, add up to the power supply voltage. Lowering Vgsoff allows the drain voltage to swing over a larger range.

Hope this helps!
 
#37
Thanks again for your schematic and your support! I just finished the project. I used two BF245C an built a two channel buffer box.
I measured the Vgsoff of the FETs and they turned out to be around 4.7V. Not bad...
I used 2.4k as Rs to reduce the power consumption (3mA idle with both channels on) and 1uF coupling foil capacitors.
I haven't played through an amp jet but test recordings directly into the soundcard with and without buffer are showing good results. Now I can use a small compact mixer with 3-band equalization and 20k input impedance to blend both channels. No need to buy an expensive two channel preamp anymore! :)
 
#38
I don't know if reviving this mummy thread is the best way to pose my question. If this won't work I'll try a seperate thread.

I just ordered the parts for this simple buffering preamp to install it in my NS WAV4 EUB. Normaly I use my phishman platinum pre, but that adds quite some color, and sometimes I want to go straight into an effect or amp.

I think I've read most of the Fdeck preamp threads and what I am wondering about is what to do with the original tone and volume control. I was planning in installing the pre after the passive tone and volume control, as i think they're doing just fine. I've seen fdeck and some others advise against this, but it's not clear to me why. Can someone please explain?
 
#39
I have a question as well. I built the basic preamp for this school year, but we had it floating instead of mounted. This summer we're mounting it on some perfboard, and I think I might be making a few additions to it at the same time. One I was particularly interested in was a blend w/ a mic. I looked at Mr. Deck's website, and found the BBBB, but I was looking for something with just the mix option and the preamp. Can anybody help me?

EDIT: The link on the OP doesn't work.
 
#40
I found a better (and cheap) replacement JFET for the PN4416 that works better (which means with less attenuation) than the BF245C: the J310.

I built a JFET-buffer with two stages (to variably mix two signals and boost them again) and using BF245C (with optimized drain/source resistors) damped the signal considerably. Replacing them with J310 made the output stronger.
I'm not sure if they sound as good as the pn4416, but they are not bad.

I paid 22 Eurocent for one J310 at Reichelt Elektronik in Germany.

Warning: Pinout is differing from the BF245C pinout. Look into the datasheet of the J310.

I used 6.8k source and drain resistors (to be able to get the inverted and non-inverted signal). Lower values (the lowest I kept in some schematics was 2.7k) will attenuate the signal by a few percent and enlarge the power consumption. Higher values don't boost the signal by any substantial amount but may limit the output current (my maximum resistance was 15k).
I have always used the same value for source and drain resistor. For single resistor designs you need to experiment yourself.

I hope this might help somebody to build FDecks wonderful boxes.
 

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