Barcus Berry - Hot-Dot Box ( preamp )

Barcode · Post by **Barcode** » 04 Sep 2008, 04:33

Fairly simple circuit, i've traced a q&d schem from the p2p insides. sound aint half bad either. the transistor is the only thing i can't figure out. it is marked "E230" but i can't find any info or datasheets on it. I was thinking it might be a jfet, but i'm not sure. any thoughts? i drew the schem according to transistor orientation since i don't know the pinout.

analogguru · Post by **analogguru** » 04 Sep 2008, 05:03

The E230 is a J-Fet from Siliconix which is electrically identical to the J230 (in another case) whose datasheet can be found here:
http://www.datasheetarchive.com/J230-datasheet.html

The Funny thing is that I have seen innards from 3 different hot-dot boxes, but all used two transistors and trimmer mounted on a pcb. I assume the second transstor is used as an output buffer there. There can also be recognized two Siemens film capacitors, so I doubt that the input cap is only 180p, please verify what is written on this cap.

analogguru

Barcode · Post by **Barcode** » 04 Sep 2008, 11:25

analogguru wrote:The E230 is a J-Fet from Siliconix which is electrically identical to the J230 (in another case) whose datasheet can be found here:
http://www.datasheetarchive.com/J230-datasheet.html

The Funny thing is that I have seen innards from 3 different hot-dot boxes, but all used two transistors and trimmer mounted on a pcb. I assume the second transstor is used as an output buffer there. There can also be recognized two Siemens film capacitors, so I doubt that the input cap is only 180p, please verify what is written on this cap.

analogguru

it says 180....

analogguru · Post by **analogguru** » 04 Sep 2008, 16:29

With 180p it acts a little bit like a treble booster as you can see in the graph below.

analogguru

Barcode · Post by **Barcode** » 04 Sep 2008, 20:34

analogguru wrote:With 180p it acts a little bit like a treble booster as you can see in the graph below.

analogguru

so then, for a more flat response, a larger cap like maybe a 4.7uF could be used on the input, right? This seems like a pretty tiny and versatile circuit, i might whip up a layout now that you have kindly enlightened me as to the datasheet...

do you think based on the datasheet that use of a 2n5457 or similar would require a rebias?

gus · Post by **gus** » 05 Sep 2008, 18:40

Why would it even need a input cap if it is for a piezo?

TragicTravisty · Post by **TragicTravisty** » 05 Sep 2008, 20:16

well it doesnt seem to be meant for protection, since it acts as a treble booster.

soulsonic · Post by **soulsonic** » 06 Sep 2008, 09:15

gus wrote:Why would it even need a input cap if it is for a piezo?

EXACTLY!!!!! This is a piezo preamp.

Barcode · Post by **Barcode** » 07 Sep 2008, 04:43

soulsonic wrote:
gus wrote:Why would it even need a input cap if it is for a piezo?
EXACTLY!!!!! This is a piezo preamp.

i don't quite understand why this means it wouldn't need an input cap. sorry, i'm a bit dense

TragicTravisty · Post by **TragicTravisty** » 07 Sep 2008, 12:25

well input caps are used to filter out lows and/or dc current. dc current doesnt need to be filtered because it is always first in the chain. bass problably wouldnt normally be cut on a pieze preamp to give the most transparent sound possible.

analogguru · Post by **analogguru** » 07 Sep 2008, 12:39

TragicTravisty wrote:....dc current doesnt need to be filtered because it is always first in the chain.

...and from where do you know that there is no DC ?
Every piezo-crystal delivers DC when stressed and bended. That it becomes AC is because the stress and bending varies with the signal.
Therefore, depending on the pressure of the strings on the piezo there could be a (small) amount of DC which would detoriate the bias of the FET.

I have to say that I never measured the DC of a piezo - maybe somebody likes to do that with a high-impedance meter.

analogguru

TragicTravisty · Post by **TragicTravisty** » 07 Sep 2008, 12:42

there you go. analogguru kicks ass, takes names, and still gives the answer.

Barcode · Post by **Barcode** » 07 Sep 2008, 15:56

analogguru wrote:
TragicTravisty wrote:....dc current doesnt need to be filtered because it is always first in the chain.
...and from where do you know that there is no DC ?
Every piezo-crystal delivers DC when stressed and bended. That it becomes AC is because the stress and bending varies with the signal.
Therefore, depending on the pressure of the strings on the piezo there could be a (small) amount of DC which would detoriate the bias of the FET.

I have to say that I never measured the DC of a piezo - maybe somebody likes to do that with a high-impedance meter.

analogguru

and, i'm just guessing here, that the small value of the input cap is directly related to the small amount of dc expected? and if this were to be used as an actual booster in a regular pedal chain, then the cap would be needed for dc blocking right?

analogguru · Post by **analogguru** » 07 Sep 2008, 16:30

and if this were to be used as an actual booster in a regular pedal chain, then the cap would be needed for dc blocking right?

You ever know what the customer does with the unit, for this reason for safety it should be placed there - DC can also occur because the output cap of the previous unit is leaky.

analogguru

Barcode · Post by **Barcode** » 07 Sep 2008, 16:46

analogguru wrote:
and if this were to be used as an actual booster in a regular pedal chain, then the cap would be needed for dc blocking right?
You ever know what the customer does with the unit, for this reason for safety it should be placed there - DC can also occur because the output cap of the previous unit is leaky.

analogguru

that's what i thought. so does the small value cap also make the unit function as a treble booster? if i were to up the input cap to say, 1 uF or maybe higher, would that broaden the frequency range of the boost?

analogguru · Post by **analogguru** » 07 Sep 2008, 17:39

Some posts above I posted a frequency response plot. If you look at it, you will see that it acts like a treble booster.

If you want to have it flat within -1,5 db down to 20 Hz 22n should be enough (with the 2M2 input resistor). For guitar 10n is more than enough.

analogguru

soulsonic · Post by **soulsonic** » 08 Sep 2008, 05:02

Maybe it has such a small input cap to compensate for the frequency response particular to the Hot Dot pickup itself?

analogguru · Post by **analogguru** » 08 Sep 2008, 06:03

Since a piezo-pickup is mainly intended to be mounted on an acoustic guitar I think it should nt compensate the pickup itself, more it should compensate the (shifted) frequency response of the guitar.

Here is the Hot Dot Box I was talking about:
http://i7.ebayimg.com/02/i/001/0a/f7/748a_1.JPG
http://i11.ebayimg.com/03/i/001/0a/f7/76b7_1.JPG

As you can see it uses a plastic transistor, a metal-can transistor, two Siemens capacitors and a Piher trimmer. Therefore I suspect that it has been built in UK.

The smaller Siemens film-cap is connected to the input with the green wire and is much higher than 47p - I suspect between 10n and 22n.

The unit can be bought here:
https://cgi.ebay.com/Vintage-Barcus-Ber ... dZViewItem

analogguru

Barcode · Post by **Barcode** » 09 Sep 2008, 00:06

analogguru wrote:Since a piezo-pickup is mainly intended to be mounted on an acoustic guitar I think it should nt compensate the pickup itself, more it should compensate the (shifted) frequency response of the guitar.

Here is the Hot Dot Box I was talking about:
http://i7.ebayimg.com/02/i/001/0a/f7/748a_1.JPG
http://i11.ebayimg.com/03/i/001/0a/f7/76b7_1.JPG

As you can see it uses a plastic transistor, a metal-can transistor, two Siemens capacitors and a Piher trimmer. Therefore I suspect that it has been built in UK.

The smaller Siemens film-cap is connected to the input with the green wire and is much higher than 47p - I suspect between 10n and 22n.

The unit can be bought here:
https://cgi.ebay.com/Vintage-Barcus-Ber ... dZViewItem

analogguru

hmm, quite a variation on my unit...

gus · Post by **gus** » 14 Sep 2008, 20:30

Something to keep in mind. What is the piezos capacitance? This will matter when making a frequency response chart like Analoguru posted. I have not measured a BB pickup so I don't know what the cap value of the pickup is.

AG how did you model the signal to the preamp? AC voltage source and what value piezo capacitance?

You also have the miller cap drain to gate that makes a fet stage like that act like a charge amp.

Barcus Berry - Hot-Dot Box ( preamp ) [schematic]