I am thinking of making a simple integrated amplifier, consisting of a Purifi power amplifier module.
My idea is to make a simple passive pre-amp with an input selector and a volume control, but do I need to think of input and output impedance on the pre-amp part, or is the impedance that the low ohm input selector, the volume control and the purify makes together?
My idea is to make a simple passive pre-amp with an input selector and a volume control, but do I need to think of input and output impedance on the pre-amp part, or is the impedance that the low ohm input selector, the volume control and the purify makes together?
The volume pot is the load for the source you wish to control.
The amplifier is the load for the volume pot.
You should keep the resistance of the volume pot high so as not to stress the source.
There's a 'sticky' on the topic here, where the experts explain it better than I can: https://www.diyaudio.com/community/...tentiometer-as-a-passive-preamplifier.219342/
The amplifier is the load for the volume pot.
You should keep the resistance of the volume pot high so as not to stress the source.
There's a 'sticky' on the topic here, where the experts explain it better than I can: https://www.diyaudio.com/community/...tentiometer-as-a-passive-preamplifier.219342/
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Thank you a lot, I'll read that. 🙂
I think to understand that the Purifi is best if the impedance is 10k Ohm and I think to understand that the input impedance in the pre-amp shall be from 47k-100k Ohm?
I think to understand that the Purifi is best if the impedance is 10k Ohm and I think to understand that the input impedance in the pre-amp shall be from 47k-100k Ohm?
I believe the argument being made in the 'sticky' is that a 47-100 k ohm pot may have too high a resistance, and that a 10 k ohm pot is preferable.
No doubt the experts will chip in!
No doubt the experts will chip in!
If Purifi says their amplifier input impedance is 10k, then a 10k pot is about the highest value suitable.
But a linear pot will work best in this case, because of the wiper loading by the amplifier input impedance.
A source will typically have less than 1k output impedance. Make sure the amplifier has enough voltage gain
to make using just a pot for a line stage workable with your speakers. Their amplifiers seem to have rather low gain.
One I saw had 12dB gain, but think they have an optional input module that adds more gain.
But a linear pot will work best in this case, because of the wiper loading by the amplifier input impedance.
A source will typically have less than 1k output impedance. Make sure the amplifier has enough voltage gain
to make using just a pot for a line stage workable with your speakers. Their amplifiers seem to have rather low gain.
One I saw had 12dB gain, but think they have an optional input module that adds more gain.
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I think it may be smartest to get a chip to adjust the volume, and let it be controlled by an analog pot. thereby, do I avoid having to use a pot that deteriorates and change the sound.
Many companies do that, since it is much cheaper than a high quality switched resistor volume control.
Here is an example, 12k input impedance with buffered output.
You'll need digital design skills to implement this.
Here is a link to a built version on ebay. No idea how good it is, but seems ok.
https://www.ebay.com/itm/185442597035
You'll need digital design skills to implement this.
Here is a link to a built version on ebay. No idea how good it is, but seems ok.
https://www.ebay.com/itm/185442597035
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Thank you, unfortunately do I not think I am able to create an A/D converter from an analog pot. I think I have to look for some analog chip? Oh and yes, I have designed it as dual mono, to keep the circuit as easy as possible to understand for me. 🙂
Look at the ebay built-up board above for an example of implementation. Again, you have to verify that your amplifier has enough gain to make this approach practical. If the Purifi amplifier really does have only 12dB gain, you will need either
their optional gain input board, or else an active line stage with rather high gain.
their optional gain input board, or else an active line stage with rather high gain.
Great point.
This is the data for EVAL2
But on the other hand is 425W insanely much for my needs, since I am sitting 6-9 feet from the speakers.
This is the data for EVAL2
| Amplifiers | 1x 1ET400A |
| Analog inputs | Balanced (XLR) |
| Speaker outputs | Banana-plug & solder point |
| Selectable pre-gain | by-pass / +13dB (front-end only) ~14dB / ~27dB (front-end + amp) |
| Power Supply inputs | ±35V to ±65V (VP) 15V (VDR) ±15V (VUNREG) |
But on the other hand is 425W insanely much for my needs, since I am sitting 6-9 feet from the speakers.
I believe that depends upon your speaker's efficiency?
I'm going Purify (EVAL1/2, VTV, etc) as well, btw. I look at the wattage as extra headroom. I have a few sets of speakers that I'll be driving; one pair 89db, another 91db, not sure about the 3rd but in that ballpark.
You may be just fine with a passive level control. When I'm using the Purify to drive nearfield monitors, I don't expect to need anything other than that 🙂
I'm going Purify (EVAL1/2, VTV, etc) as well, btw. I look at the wattage as extra headroom. I have a few sets of speakers that I'll be driving; one pair 89db, another 91db, not sure about the 3rd but in that ballpark.
You may be just fine with a passive level control. When I'm using the Purify to drive nearfield monitors, I don't expect to need anything other than that 🙂
To be honest, have I absolutely no idea about the gear around my amplifier, I may change one thing, build another, redesign the amp and so on.
I suspect to build some speakers that can have whatever value, but no matter what is the volume I usually listen to, from about 60dB to 80dB and the speakers are very near to me (6-9 feet)
I suspect to build some speakers that can have whatever value, but no matter what is the volume I usually listen to, from about 60dB to 80dB and the speakers are very near to me (6-9 feet)
Then you should have a line stage with 20dB gain, that also has an optional buffered output for unity gain.
That will cover all bases.
That will cover all bases.
If R1 and R2 are parts of the pot then the source output impedance was not considered. If R1 is considered the source output impedance then the pot series resistance was not considered. Anyhow just add the value of the source output impedance to R1 since they are in series and that equation will still be correct. The series-parallel equivalent resistance value of the source output impedance + the pot series + pot shunt//power amp input impedance) will be the total load to the source.
If we look at my idea as a super simplified module by module, may it be easier for me to explain it correctly and tells you on what level I understand things.
(The resistors have I called R20 and R21 to not confuse them with other resistor numbers from examples you may give where there are other resistors)
Dual mono:
Input selector (NO reed-relay to next step in the pre-amp) - (NC SSR, followed by R21 to short the + on the music source to GND)
R20 can maybe be used to add input impedance.
Volume control mono pot-chip
Buffer to change output impedance (is passive possible?)
Output to Purifi-amplifier module.
(The resistors have I called R20 and R21 to not confuse them with other resistor numbers from examples you may give where there are other resistors)
Dual mono:
Input selector (NO reed-relay to next step in the pre-amp) - (NC SSR, followed by R21 to short the + on the music source to GND)
R20 can maybe be used to add input impedance.
Volume control mono pot-chip
Buffer to change output impedance (is passive possible?)
Output to Purifi-amplifier module.