Is this possible from a passive pre-amp?

mr.duck · 2nd December 2006, 04:03 PM

I came across a passive pre-amp that has 15k ohm input impedance and a MAXIMUM output impedance of 3750 ohms. I cannot see how that is possible prom a passive pre amp. If it's just a basically a pot, then when the volume is turned down very low, output impedance is going to be roughly equal to the value of the pot in series with the output impedance of the source.

If that is possible then perhaps passive pre-amps are not so bad after all. But does anyone know how it's done? I feel like I am missing something.

darkfenriz · 2nd December 2006, 04:07 PM

Transformer?

mr.duck · 2nd December 2006, 04:26 PM

hmmm could be. I can't find any pictures of the insides. It's a rothwell passive preamp. Seriously expensive (£800). I can't believe that having a transformer in there would be better than a simple buffer using transistors.

Is there anything else it can be apart from a transformer?

audio-kraut · 2nd December 2006, 04:40 PM

Quote:

s there anything else it can be apart from a transformer?

Magic? After all - the land of hairy pot'er...

Geoff · 2nd December 2006, 04:59 PM

Quote:

Originally posted by mr.duck
I came across a passive pre-amp that has 15k ohm input impedance and a MAXIMUM output impedance of 3750 ohms. I cannot see how that is possible prom a passive pre amp. If it's just a basically a pot, then when the volume is turned down very low, output impedance is going to be roughly equal to the value of the pot in series with the output impedance of the source.

If that is possible then perhaps passive pre-amps are not so bad after all. But does anyone know how it's done? I feel like I am missing something.

Not only is this possible, it is a certainty. The maximum output impedance of a potentiometer occurs when it is mid-travel (electrically not mechanically if it is a log pot). If the source has an output impedance close to zero then effectively you have the two halves of the track in parallel. As each half has a resistance of 7k5, the resulting output impedance will be 3k75. The output impedance will fall towards zero as the potentiometer is rotated towards its maximum or minimum positions.

JesseG · 3rd December 2006, 04:47 AM

Thank you Geoff

I love to learn things and I study audio stuff frequently but I still cannot figure out impedance!

Jess

AndrewT · 3rd December 2006, 08:16 AM

Hi Geoff,
you got it just right.

darkfenriz · 3rd December 2006, 11:57 AM

Stupid me....

2nd December 2006, 04:03 PM	#1
mr.duck diyAudio Member Join Date: Jun 2006	Is this possible from a passive pre-amp? I came across a passive pre-amp that has 15k ohm input impedance and a MAXIMUM output impedance of 3750 ohms. I cannot see how that is possible prom a passive pre amp. If it's just a basically a pot, then when the volume is turned down very low, output impedance is going to be roughly equal to the value of the pot in series with the output impedance of the source. If that is possible then perhaps passive pre-amps are not so bad after all. But does anyone know how it's done? I feel like I am missing something.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Passive Crossover for Passive Subwoofer?	Toast_Master	Subwoofers	23	30th April 2006 12:14 AM
passive pre amp	mschwilson	Solid State	10	27th July 2005 01:43 PM
dB loss by using passive crossovers? Active vs Passive and 1st vs 4th order	Hybrid fourdoor	Multi-Way	3	11th July 2004 09:16 PM
Help with passive pre	sejarzo	Everything Else	9	27th April 2004 06:11 PM
Passive EQ?	Johan_Ch	Everything Else	1	4th September 2003 04:11 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

2nd December 2006, 04:07 PM	#2
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	Transformer?

2nd December 2006, 04:26 PM	#3
mr.duck diyAudio Member Join Date: Jun 2006	hmmm could be. I can't find any pictures of the insides. It's a rothwell passive preamp. Seriously expensive (£800). I can't believe that having a transformer in there would be better than a simple buffer using transistors. Is there anything else it can be apart from a transformer?

3rd December 2006, 04:47 AM	#6
JesseG diyAudio Member Join Date: Jul 2005 Location: Mid coast of British Columbia	Thank you Geoff I love to learn things and I study audio stuff frequently but I still cannot figure out impedance! Jess

3rd December 2006, 08:16 AM	#7
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi Geoff, you got it just right.

3rd December 2006, 11:57 AM	#8
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	Stupid me....