[AudioI] Guitar Preamp for Isolated(Pro) soundcard

[Ted Rippert]

Joseph,
A buffer with an input impedance of 100k is very good for a guitar/bass with active electronics as it does reduce the effect of stray capacitance in the cable. However with passive pickups and no preamp in the guitar, this is too low an input impedance.  In that case you want at least around 500k and preferably 1Meg. This does cause more roll off due to the cable capacitance, but the loading on the pickups is far worse for tone with a low impedance input. This is why many amplifiers and audio interfaces have a low and a high impedance input (or a switch that changes the impedance) to accommodate both active and passive instruments. This actually may be a good idea for a possible instrument preamp for the Pro card, though it does increase the parts count.

As a side note, while the cable capacitance does cause some roll off of high frequencies, many guitarists prefer this tone. I actually have a wireless unit that has one output that has “cable tone compensation” which is just a low pass filter, and another output that does not.

> On Mar 4, 2022, at 6:08 AM, Joseph DiVerdi <diverdi at xtrsystems.com> wrote:
> 
> Yep, it's a great differential driver. I would not recommend using it as an input circuit directly, however. I would only rarely use such a large input (and feedback) resistor because the inevitable stray capacitance(s) will ruin the otherwise good performance of the circuit by "rolling off" the high frequencies without their being under your control.
> 
> If you're looking to drive this circuit directly from a "guitar output", that is, long, shielded, high impedance (>100k) cable, low signal (low hundreds of mVpeak) then a one more op-amp in a non-inverting configuration with a single resistor to ground (hundreds of k) and gain in the feedback look if you require it is more robust. (see input buffer figure)
> 
> jadv
> 
> On Fri, 04 Mar 2022 2:20 AM, Ted Rippert wrote:
>> Thanks. I have seen this circuit before, but forgot about it. It could make a nice input circuit if you change the 10k input resistor for a 1Meg and set the gain with an appropriate value of feedback resistor instead of the 10k there. The codec has a common mode voltage output that could be used to set the bias point. Should also be possible to add a small Capacitor in the feedback loop to get the filtering that the data sheet asks for.
>>> On Mar 3, 2022, at 6:31 PM, Joseph DiVerdi <joseph.diverdi at colostate.edu <mailto:joseph.diverdi at colostate.edu>> wrote:
>>> 
>>> Here's an example of a circuit that can accomplish what you want.
>>> 
>>> <image0.png>
>>> If IC1and IC2A are eliminated you have a robust single to differential converter. Easy resistor values. If you use bipolar power you can take the plus terminal of IC3B to ground in the outputs will be centered around 0V. It's DC coupled also. You can use the op amps of your choice. I'm partial to the Analog Devices ones because extremely high quality  along with good prices. But like I say you can use whatever ones you like.
>>> 
>>> jadv
>>> 
>>> 
>>>> On Mar 3, 2022, at 5:55 PM, Ted Rippert via People <people at lists.audioinjector.net <mailto:people at lists.audioinjector.net>> wrote:
>>>> 
>>>> Lots of good information so far, but I do want to get back to the main purpose of this thread. Making a preamp specifically for the Pro sound card.
>>>> 
>>>> Since the Pro already has +-12VDC on it, I’ll assume that the power supply is taken care of.
>>>> 
>>>> One thing I would like to get more information on is a good opamp circuit to interface a passive pickup to the differential inputs available on the Pro card header. The CS4272 data sheet gives an example of a circuit to use for the analog input, but it is differential to differential, and AC coupled. There is also a recommended output circuit that is AC coupled. The specs on the Pro card and the existing Preamp boards for it say that they are DC coupled. I like the idea of DC coupling, especially for a Bass, but what are the technical details. The data sheet does state that you need to filter out 6.144 MHz and it’s harmonics, which is fairly straight forward with an opamp. There are various differential to single ended and vice-versa circuits documented on the web, such as:
>>>> 
>>>> 
>>>> 
>>>> Does anyone have experience into how to modify such circuits for a high impedance input while maintaining the needed filtering and possibly adding a gain control on the input? I know the theory on how to do such things, but there are a lot of possible options and the devils are always in the details. Also, are the pins on the Pro card header tied directly to the Codec chip, or is there some circuitry in between? I’m going to order a Pro board and one of the preamp boards for general use and to see what I can find out about the circuits and layout. Just hoping to get a bit of a head start here.
>>>> 
>>>> Thanks, and I don’t mean to stop any of the other conversations in this thread. They are very useful to me and hopefully others. Just want to also try to get some info on the main subject.
>>>> -Ted
>>>> 
>>>>> On Mar 2, 2022, at 2:41 PM, Ted Rippert <ted.rippert at gmail.com <mailto:ted.rippert at gmail.com>> wrote:
>>>>> 
>>>>> Hello,
>>>>> I’ve seen several threads on this list relating to using Guitar or Bass with an Audio Injector card. I’ve been doing that for years using a rack mount mixer with a couple High-Impedance inputs interfacing into an Audio Injector Stereo card. Works very well, and I can get round trip latencies on the Pi as low as 2.1 ms running Guitarix amp sim if I use some CPU isolation techniques.
>>>>> 
>>>>> I’ve been playing around with making a small preamp/buffer that runs on the 5V from the Pi, and found just what others have, that it’s too noisy. So I worked up a setup with a dual supply converter that then goes into a couple linear regulators. It does work, but it’s a mess using just perf board and through hole parts. Also, the Zero sound card I used has the same usable but high noise floor that the stereo card does.
>>>>> 
>>>>> So I want to try making a preamp for the Isolated card (really a Pro card from Amazon, but same interface). I would like to talk to Matt and any others with experience about the design of such a card, and any possibility of an “official” Audio Injector version being made. I just do perf board/through hole stuff, so I can only make one-offs myself.
>>>>> 
>>>>> To get things started, let me list some specs for such a card:
>>>>> 
>>>>> Input impedance: > 500kohm, 1 Mohm nominal
>>>>> Input voltage: ~ 1 VDC pk-pk, preferably with an adjustable gain and significant headroom
>>>>> Input connector: 1/4 inch TS socket
>>>>> Output impedance: low audio interface level, preferably less than 300 ohms
>>>>> Output voltage: at least +4dBu, preferably with adjustable gain, since the codec has an adjustable analog output volume, this may be in software
>>>>> Output connector: 1/4 inch socket, preferably a TRS type that will work as both a balanced and unbalanced output depending on what is connected to it
>>>>> 
>>>>> It could be as simple as a modified RCA preamp with different connectors and higher impedance input circuits, or you can think about adding things like a DIN based MIDI interface and a headphone amplifier output. All of this seems quite doable using the AI-ABS standard, but I need some more details about the specs of that, and about the practicalities of how to design opamp buffers to interface to the codec on the Pro board.
>>>>> 
>>>>> So, if anyone is interested, lets talk about it.
>>>>> 
>>>>> Thanks,
>>>>> Ted
> <input buffer.jpg>