Hi folks,
I had the chance to build the FE2022. Thank you @6L6 for all the help. Papa's instructions were sufficient except for one small bit of confusion, which is to say, you must connect ALL of the return paths together (you'll see the jumper wire in my pictures).
I built the single supply, single-ended version.
You will notice that this project, like most of the Pass preamps, will fit into the usual chassis from the store. I was fresh out and wasn't sure if I wanted to keep this in a dedicated box, so I repurposed my B1 buffer and as you can see, things swapped in pretty nicely. This also makes A/B/C testing / comparisons quite straight-forward and will rule out other variables as you are only swapping out the circuit - the vol pot/switch/PSU can be the same. You can even power two directly from the same PSU.
All in all, the circuit tested as expected with the following gain values when I swapped R4/3. Note that my values for gain/db are approximate to say +/-1db and +/- .5 on gain.
All in all, the circuit sounds nice for quiet passages at the two lower values of gain but I could start to notice distortion at higher volume levels with the 33k R. Nothing awful by any means, but I could notice. I will say that at the two higher gain levels, this circuit would make for a nice "clean boost" guitar / FX pedal.
I had the chance to build the FE2022. Thank you @6L6 for all the help. Papa's instructions were sufficient except for one small bit of confusion, which is to say, you must connect ALL of the return paths together (you'll see the jumper wire in my pictures).
I built the single supply, single-ended version.
You will notice that this project, like most of the Pass preamps, will fit into the usual chassis from the store. I was fresh out and wasn't sure if I wanted to keep this in a dedicated box, so I repurposed my B1 buffer and as you can see, things swapped in pretty nicely. This also makes A/B/C testing / comparisons quite straight-forward and will rule out other variables as you are only swapping out the circuit - the vol pot/switch/PSU can be the same. You can even power two directly from the same PSU.
All in all, the circuit tested as expected with the following gain values when I swapped R4/3. Note that my values for gain/db are approximate to say +/-1db and +/- .5 on gain.
| R1(2) | R4(3) | Gain | db |
| 10k | 100k | ~10 | ~20db |
| 10k | 47k | ~4.7 | ~13db |
| 10k | 33k | ~3.3 | ~10db |
| 10k | 22k | ~2.2 | ~6.8db |
| 10k | 18k | ~1.8 | ~5db |
All in all, the circuit sounds nice for quiet passages at the two lower values of gain but I could start to notice distortion at higher volume levels with the 33k R. Nothing awful by any means, but I could notice. I will say that at the two higher gain levels, this circuit would make for a nice "clean boost" guitar / FX pedal.
Got a question about using the DIYFE22 for phase reversal. The article by Mr. Pass shows how to use it single ended by grounding the -IN input and using the +IN input attached to the music source. Is the reverse also true? Can I ground the +IN input and connect the -IN input to my source to get a phase-reversed signal at the output?
I'm pulling things together to build my DIYFE22 boards. Looking to use them for se to blanced conversion with sockets to sort out the gain for bridging. I've noticed the BOM specifies .5 watt resistors. I'm good for the 10k resistors, but I'll be ordering the rest that aren't in the essentials kits. 6L6's build and the resistors supplied with the jfets all look like .25 watt or so. Should I look for .5s?
Thanks, Skip
Thanks, Skip
The only resistors that dissipate a bit of heat are R11 and R12 at a bit less than 0.1W. The rest of the resistors dissipate in the single digit mW range or less (less than 10mW or 0.01W). So 1/4W resistors will be fine, and you can use your 1/2W 10k for R11 and R12,
I'll add that when shopping for resistors one should check on the size of the parts as well. If you search just on the dissipation rating you could end up with, for example, some RN65 resistors which are rated at 0.5W (mil-spec), but are likely too large for the boards.
Given this particular PCB, I suggest filtering for resistors with lengths, say, less than 7 or 8mm.
Given this particular PCB, I suggest filtering for resistors with lengths, say, less than 7 or 8mm.
Hi
A question to increase my knowledge / understanding:
in Nelson's article ("The circuit is designed to operate either as a line level gain stage with a maximum output of 15V rms or as the front end of a modest follower output stage")
and proven by the many example of the front end being used as a preamp, what is the difference between a preamp and the front end of a power amplifier ? Is it that a preamp will make sure to be agnostic to the next element to be connected to it ?
A question to increase my knowledge / understanding:
in Nelson's article ("The circuit is designed to operate either as a line level gain stage with a maximum output of 15V rms or as the front end of a modest follower output stage")
and proven by the many example of the front end being used as a preamp, what is the difference between a preamp and the front end of a power amplifier ? Is it that a preamp will make sure to be agnostic to the next element to be connected to it ?
An amplifier FE (front end) typically consists of an IPS and VAS (input stage, voltage amplification stage). It's engineered to supply the OPS (output stage) adequate voltage swing and gain to meet the design goals. The OPS is typically current gain only, no voltage gain there. Sometimes an FE can make for a fine general purpose "gain stage" as a building block in a stand alone "preamplifier".
A preamplifier can take many forms. It may provide voltage gain, or it may not (unity gain), depending on the designers intent. These typically have volume control, input switching, maybe a balance control, maybe a built-in phono stage, and so on. These will usually have an output stage that allows for driving more difficult loads, for example interconnect cables with significant capacitance or even headphones.
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woody - 9mA was mentioned earlier in the thread. I have not finished building mine yet so I can't confirm.
Thanks @william2001.
I was focused on the electronic part, so "Sometimes an FE can make for a fine general purpose "gain stage" is the answer regarding the electronic part, whereas I forgot the inputs selection / volume control, because I see it as more "mechanical". But thank you for the detailed answer.
& merry Xmas to all!
I was focused on the electronic part, so "Sometimes an FE can make for a fine general purpose "gain stage" is the answer regarding the electronic part, whereas I forgot the inputs selection / volume control, because I see it as more "mechanical". But thank you for the detailed answer.
& merry Xmas to all!
electronic part - A front end is designed to basically just drive the gate / base of some driver or output transistors, and do that job very well. There may be some other real world loads where this FE won't be happy without some "help", in the form of a buffer stage (normally a "follower") on the output. Look at the last stage of the BA2018, WHAMMY, or Pearl 3 as a few quick examples..
Merry Xmas!
Merry Xmas!
Hi everyone,
I am not an electronics engineer and English is not my first language. So a thousand apologies if I say nonsense.
I see preamps with a differentiel input, based on JFET transistor like the 2SK170 or dual ones like the LSK389.
But what is the advantage of using discrete transistors when chips such as OPA1642 have a Jfet differential input and very nice performances ?
Are there real technical reasons ?
Thank you for your answers
Alan
I am not an electronics engineer and English is not my first language. So a thousand apologies if I say nonsense.
I see preamps with a differentiel input, based on JFET transistor like the 2SK170 or dual ones like the LSK389.
But what is the advantage of using discrete transistors when chips such as OPA1642 have a Jfet differential input and very nice performances ?
Are there real technical reasons ?
Thank you for your answers
Alan
