Can someone assist me in working out resistance pairs for specific fixed gain/attenuation.
In my application I don't need variable volume control, as that is provided elsewhere, but I'd like to have three specific levels, the maximum attenuation will be either -10dB or -20dB - I'd then like 0dB, and a gain setting of +20dB but reading the paper, it looks like only +18dB is on offer, so I'd maybe go for +15dB to be "safe"
Is there some way of working out the gain for sets of resistances, i.e. if it's 1k on the left side of the "wiper" and 9k on the right, what is that in dB terms. My maths is ok for add and subtract, but logarithmic scales like dB just cause a slight burning smell in my brain, so if anyone can help me, it would be much appreciated.
I could stick a formula into a spreadsheet, if someone can offer me a formula!
In my application I don't need variable volume control, as that is provided elsewhere, but I'd like to have three specific levels, the maximum attenuation will be either -10dB or -20dB - I'd then like 0dB, and a gain setting of +20dB but reading the paper, it looks like only +18dB is on offer, so I'd maybe go for +15dB to be "safe"
Is there some way of working out the gain for sets of resistances, i.e. if it's 1k on the left side of the "wiper" and 9k on the right, what is that in dB terms. My maths is ok for add and subtract, but logarithmic scales like dB just cause a slight burning smell in my brain, so if anyone can help me, it would be much appreciated.
I could stick a formula into a spreadsheet, if someone can offer me a formula!
1K on the left and 9K on the right gives 20*log(9000/1000) = +19 dB gain, and so on and on.
There is no real limit on the gain, so 0.9K on the left and 9k1 on the right gives +20dB, the other way round gives -20dB.
Hans
There is no real limit on the gain, so 0.9K on the left and 9k1 on the right gives +20dB, the other way round gives -20dB.
Hans
Many thanks, Hans - I wasn't expecting it to be that simple! So, 5k on both sides really is unity gain, and 0k / 10k gives either complete attenuation, or infinite gain... neat and tidy.
Another related question - how critical is the 10k figure? If I had a set of resistors that added up to, say 12k, or 8k would that matter much?
Great, many thanks... I can now build my two relay three gain "pot" - I'll play about with some resistors on a spreadsheet.
while you could set up a spreadsheet, it is the DIY way, you could also use on of the calculators on line, such as
inverting op-amp gain calculator
Cheers
Alan
inverting op-amp gain calculator
Cheers
Alan
I got a lot more at the two extremes of the lin vol pot.
max gain exceeded +40dB and max attenuation exceeded -40dB.
The total range from min to max was approximately 90dB over the 270degrees of vol pot electrical rotation.
if you wanted to limit gain range, you would add two resistors, of equal value if you want to maintain symmetry, one to each side of the pot.
adding 900 ohms to each side limits the gain range to a nominal plus or minus 20 db.
Cheers
Alan
adding 900 ohms to each side limits the gain range to a nominal plus or minus 20 db.
Cheers
Alan
I'm going for a 1k resistor at each end, and two 5k6 between them. I'm going to connect the "wiper" connection between the two 5k6 resistors, this will give me 0dB.
I will then use two relays to short out either of the two 5k6 resistors and this will give me either +16 or -16dB, as I only need three gain settings.
I am right in assuming that the quality of these resistors are non critical? I do have some suitable vishay bulk foils, but I also have some standard metal films, which I will use.
I will then use two relays to short out either of the two 5k6 resistors and this will give me either +16 or -16dB, as I only need three gain settings.
I am right in assuming that the quality of these resistors are non critical? I do have some suitable vishay bulk foils, but I also have some standard metal films, which I will use.
The inverting opamp uses feedback across the identical current density sides of the gain/attenuation resistors to cancel distortion due to errors in the resistors.
Once you move away from identical current density the resistor quality becomes very important
The resistors being of different value/current density need to be very high quality. Absolute minimum is medium tempco metal film. Try 1%, 50ppm/C
For better quality look at some 5, 10, or 25ppm/C, or go to folded metal foil at extortionate cost.
At the low voltages seen in a line level attenuator voltage coefficient is not critical.
Once you move away from identical current density the resistor quality becomes very important
The resistors being of different value/current density need to be very high quality. Absolute minimum is medium tempco metal film. Try 1%, 50ppm/C
For better quality look at some 5, 10, or 25ppm/C, or go to folded metal foil at extortionate cost.
At the low voltages seen in a line level attenuator voltage coefficient is not critical.
I just ordered that Vicol Audio (tibi) Maya remote volume controller for the BPBP (replacement for the linear pot) at the currently reduced price (sale!), but because the VoICB PCB, which is included, comes unpopulated I have a problem because the sum of parts is inherently rather low in monetary value, thus all the larger electronic suppliers like Mouser etc. charge a "fortune" for sending this out to the EU.
Any idea ?
Any idea ?
I combined the BPBP compoents with other builds to hit the "free postage" limit.
Digikey and Mouser have now each set up a European distribution centre, so that they now can meet their "promise" :
All taxes paid.
I didn't get a man on the doorstep demanding payment before he would hand over the parcel this time.
Digikey and Mouser have now each set up a European distribution centre, so that they now can meet their "promise" :
All taxes paid.
I didn't get a man on the doorstep demanding payment before he would hand over the parcel this time.
It costs (sale, assembled but no parts for the VoICB included) actually appr. EUR 180 incl. shipment to the EU, which is not "cheap" in the real sense in my opinion... Will test it together with the other one I ordered already from China.
I know, because that has been always the case when I ordered from Mouser before.
But the net value of the order has to reach EUR 50 (?), exluding VAT, to get beyond those EUR 20 shipping and handling charges. That bothers me...
Have a check for other suppliers in your country. There are a couple in the UK at least that do free 24hour delivery for orders over £20 or so. Farnell for example has the same offer in other countries for orders over €30, next day delivery.
Check the BOM with their stock: Farnell element14 | Electronic Component Distributors
Hello BPBP users
I finally bought the following parts: BPBP, Hans Polaks VolCB pcb and the Maya controller. The only thing I'm missing is the nice 4 input extension board that Mr. Polak designed. I searched in the different threads about BPBP but could not find any information how to get it. Do you guys know how I can get this nice extension?
Kind Regards and Thanks in advance for any help
Phil
I finally bought the following parts: BPBP, Hans Polaks VolCB pcb and the Maya controller. The only thing I'm missing is the nice 4 input extension board that Mr. Polak designed. I searched in the different threads about BPBP but could not find any information how to get it. Do you guys know how I can get this nice extension?
Kind Regards and Thanks in advance for any help
Phil
I checked the German website of Farnell, but the minimum order value for free shipment is even higher: EUR 55 😡
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