Note: I posted a query similar to this on AudioKarma; hoping more eyes will see it here.
I'd like to build a simple passive preamp, similar to some of the projects on this site, but there's a problem: my Audio Note Dac One has a high output impedance - around 6.3k. I suspect this will be problematic, especially as I have to drive a 4 meter interconnect run.
So... and forgive my ignorance if this is an unbelievably stupid idea... but could I put a step down transformer between the input and potentiometer, such that the AudioNote will always "see" 100k ohm impedance, but the potentiometer will see something nice and low?
For example, looking at Jensen microphone transformers on this page: JENSEN TRANSFORMERS, INC. - MICROPHONE INPUT TRANSFORMERS, I'm assuming that, using a JT-11K8-APC, my DAC will see 150kohm impedance; my potentiometer will see a 1.8 kohm impedance. At 47$ each, two of them, plus decent alps potentiometers, are much cheaper than a "true" autoformer type volume control, such as those from Prometheus or S&B.
The "main in" on my Yamaha CR3020 is 100kohm - as high as tube amp, so that should help, at least. Also, my system is a pair 88db efficient bookshelves in bedroom with low sloping ceillings; gain shouldn't be a problem.
It would be dual mono; I need balance adjustment, because of uncorrectable room problems.
Another question is, what happens if I put another pair of the same transformers on the output side - will be potentiometer always see 150Kohms, and my amp, 1.8kohms?
TIA....
I'd like to build a simple passive preamp, similar to some of the projects on this site, but there's a problem: my Audio Note Dac One has a high output impedance - around 6.3k. I suspect this will be problematic, especially as I have to drive a 4 meter interconnect run.
So... and forgive my ignorance if this is an unbelievably stupid idea... but could I put a step down transformer between the input and potentiometer, such that the AudioNote will always "see" 100k ohm impedance, but the potentiometer will see something nice and low?
For example, looking at Jensen microphone transformers on this page: JENSEN TRANSFORMERS, INC. - MICROPHONE INPUT TRANSFORMERS, I'm assuming that, using a JT-11K8-APC, my DAC will see 150kohm impedance; my potentiometer will see a 1.8 kohm impedance. At 47$ each, two of them, plus decent alps potentiometers, are much cheaper than a "true" autoformer type volume control, such as those from Prometheus or S&B.
The "main in" on my Yamaha CR3020 is 100kohm - as high as tube amp, so that should help, at least. Also, my system is a pair 88db efficient bookshelves in bedroom with low sloping ceillings; gain shouldn't be a problem.
It would be dual mono; I need balance adjustment, because of uncorrectable room problems.
Another question is, what happens if I put another pair of the same transformers on the output side - will be potentiometer always see 150Kohms, and my amp, 1.8kohms?
TIA....
My head hurts!
Why don't you give us a schematic of what you would like to do (do not forget in-out impedances), Someone here will help you. E
Why don't you give us a schematic of what you would like to do (do not forget in-out impedances), Someone here will help you. E
I think I would re-think your plan. 🙂
First off, I doubt your DAC has a 6.3k ohm output impedance. Where did you get that measurement? More likely less than half that. Someone who knows what they're doing should probably measure it for you.
Even so, I would move your DAC closer to the receiver so you could minimize interconnect length, and then use some sort of a buffered, active preamp to provide the interface/volume-control between the two.
Passive "preamps" have their place, but this seems like the worst possible application someone could dream up. 🙂
Cheers,
Dave.
First off, I doubt your DAC has a 6.3k ohm output impedance. Where did you get that measurement? More likely less than half that. Someone who knows what they're doing should probably measure it for you.
Even so, I would move your DAC closer to the receiver so you could minimize interconnect length, and then use some sort of a buffered, active preamp to provide the interface/volume-control between the two.
Passive "preamps" have their place, but this seems like the worst possible application someone could dream up. 🙂
Cheers,
Dave.
Mic trafos are 150 ohms, not 150k. If it really is 6.3k on the output I would forget passive and add a buffer like the B1.
Using step-down transformers is regularly done in professional audio. There are lots of passive DI boxes which can sound pretty decent.
As a test I would experiment with a cheaper transformer, like the NEUTRIK NTE4 (availlable here in Europe for about 10Euro). This transformer has a ratio of 1:4 which should be sufficient here. Stepping down 4:1 also lowers the output level also about 12db, so you would need a good lownoise amp with lots of gain.
As a test I would experiment with a cheaper transformer, like the NEUTRIK NTE4 (availlable here in Europe for about 10Euro). This transformer has a ratio of 1:4 which should be sufficient here. Stepping down 4:1 also lowers the output level also about 12db, so you would need a good lownoise amp with lots of gain.
According to this... http://www.audionote.co.uk/downloads/zero_manuals/AN_DAC_One.1x.pdf ...the output impedance is 'less than 2K Ohms', and signal level is 2.5 V RMS.
Remember that a transformer transforms both voltage and impedance. A 4:1 transformer gives a 4:1 voltage drop and a 16 (i.e. 4 x 4) : 1 impedance drop.
So a 4:1 hooked onto the output of your DAC will
... turn your 2K Ohms output of DAC into 125 Ohms output of transformer
... turn your 2.5 V output of DAC into 0.625 V output of transformer
Do NOT use a microphone transformer on this type of line level application, for they are designed to handle very low level signals and will distort very badly at line level input. The clue is in the Jensen information where it says Max Input = -2 dBu. That equates to about 0.615 V, so 2.5 V is not appropriate.
I echo the suggestion of a simple buffer such as the B1.
Remember that a transformer transforms both voltage and impedance. A 4:1 transformer gives a 4:1 voltage drop and a 16 (i.e. 4 x 4) : 1 impedance drop.
So a 4:1 hooked onto the output of your DAC will
... turn your 2K Ohms output of DAC into 125 Ohms output of transformer
... turn your 2.5 V output of DAC into 0.625 V output of transformer
Do NOT use a microphone transformer on this type of line level application, for they are designed to handle very low level signals and will distort very badly at line level input. The clue is in the Jensen information where it says Max Input = -2 dBu. That equates to about 0.615 V, so 2.5 V is not appropriate.
I echo the suggestion of a simple buffer such as the B1.
frperdurabo,
I use a passive stepped attenator, also have used a passive shunt attenuator with excellent results. I understand the principal pretty well, and like already said, it has it's place. In my little system, it's great and all I need.
But, in your situation I would not recommend it as I think you would be very disappointed.
However, here's something you may want to consider. Look into the Pass B1 buffer/zero gain preamp. In it's diy form, it's an easy build and low cost factor. It's purpose is to solve those impedance matching problems. It also allows for the use of longer interconnects. And, it uses two volume pots so balance is taken care of in that way.
If you don't want to build anything, then look into a Burson Buffer. Here's a few links to the B1 buffer.
The First Watt Model B1 Buffer Preamplifier Operating at the speed limit! Review By Dick Olsher
http://www.audiocircle.com/index.php?topic=78312.0
I use a passive stepped attenator, also have used a passive shunt attenuator with excellent results. I understand the principal pretty well, and like already said, it has it's place. In my little system, it's great and all I need.
But, in your situation I would not recommend it as I think you would be very disappointed.
However, here's something you may want to consider. Look into the Pass B1 buffer/zero gain preamp. In it's diy form, it's an easy build and low cost factor. It's purpose is to solve those impedance matching problems. It also allows for the use of longer interconnects. And, it uses two volume pots so balance is taken care of in that way.
If you don't want to build anything, then look into a Burson Buffer. Here's a few links to the B1 buffer.
The First Watt Model B1 Buffer Preamplifier Operating at the speed limit! Review By Dick Olsher
http://www.audiocircle.com/index.php?topic=78312.0
Last edited:
- Status
- Not open for further replies.