Lightspeed Attenuator a new passive preamp

Jump to Latest
A

AndrewT

R.I.P.
Joined 2004
and Bob Pease wrote this
But to get high ZOUT, you usually do have to trim.

For a 1-mA output (all R’s = 10k ±1%), the ZOUT might be as poor as 0.25 MO, +0.25 MO or even –0.25 MO. So, you have to trim. (See the little trim on Figure 2. Even 0.1% resistors would only provide a moderate improvement, to ±2.5 MO.) That App Note also shows how to get high ZOUT without any pots.
Click to expand...
 
W

wapo54001

Member
Joined 2008
It appears that I misspoke in my remarks about the AD8277 -- at 5V rail it is current limited to 10ma, not 15ma as I said earlier. This I am gathering from a re-read of the spec sheet.

However, this is still fine for my purposes. For LDRs simulating a 10K pot driving a 10K or larger load, the series LDR doesn't have to go below 200 ohms, and for the shunt LDRs, two LDRs in series/parallel at 10ma or less will deliver 25 ohms. When I tested a dozen LDRs some time ago, ten out of twelve reached 50 ohms at less than 10ma; only two required more current for 50 ohms. Those two can be used as series LDRs. This means that almost all LDRs should be usable in my circuit design, but a little selection will be necessary to insure that all shunt LDRs will achieve 50 ohms or less at 10ma or less.

I really like it that the entire circuit can be operated at 10ma or less because that means the LDRs will never come even close to being stressed enough to change performance characteristics due to heavy current passing through the LEDs.
 
W

wapo54001

Member
Joined 2008
AndrewT said:
Change R1 to set the new maximum output current.

Io(max) = Vref(max) / R1
Click to expand...

Not sure if you're saying the current can be higher than 10ma -- page 4 of spec sheet: Short-circuit current limit of +/-10ma for Vs = +2.7V to <+/-5V. Previously, I had been looking at the spec for Vs above +/-5V.

I think that means that in order to use this chip to drive an LDR "pot," you have to use two LDRs in the shunt leg in order to achieve any reasonable minimum resistance. Easily done with Vs > 4V and a current-controlled circuit.
 
Last edited:
C

costis_n

Member
Joined 2008
I am planning to make a passive pre with 100K input resistance.
It will be driven by normal, mpdern sources (DAC, Tuner & Phono pre)
It will drive a valve input stage with an east 220K input resistance.
The source to in to out cables will be less than 1 meter , with around 140pf capacitance each (shielded twisted pair).

Am I okay regarding high freq. response?
Should I modify the input resistance of the active strage?

Thanks to all
 
A

AndrewT

R.I.P.
Joined 2004
the 220k input impedance will work very well with a Source impedance of <= 220k/20, i.e. <=11k.
A 100k vol pot has a range of output impedance of [100k +Rs]/4 to zero ohms.
if your Source has an output impedance of 550r then the vol pot presents a maximum source impedance to the valve amp of [100k+500r]/4 = 25k125.

That exceeds 11k by quite a margin.
You will get good signal transfer if the ratio of input impedance:source impedance is ~10:1 and gets pooer by the time you are down to 5:1.
You 100k with the valve amp gives 220k:25k ~ 9:1
Usable, but not ideal.
 
djn

djn

Member
Joined 2004
Hi George, is your type of volume control affected by other components in the preamp such as B+ and such? At first I was thinking that I would do a stand along light control V-pot and selector switch but my audio rack is running out of space. I am now thinking of replacing the Alps pot in my preamp with the lightspeed style and use the Alps pot for controling the 5v. How isolated does it have to be?
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
Hi DJN, the only time I have seen any effect of extra noise on the scope with the ldr's of the production Lightspeed Attenuator, is when a switchmode wall wart was used, this raised the noise level double. But it is still very low (micro volts), and still ok to use if a linear cannot be found.
This noise was probably radiated from the entry wires into the internal second stage 5v regulation, as the power supply floats as is isolated it could not have come in through the audio ground plane.

Cheers George
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
You can do this Joey, but you have to be very careful of temperature changes especially if the integrated or pre has tubes as this may not allow you to keep them in a matched arrangement. In the production Lightspeed Attenuator I pot all ldr's together in hard wax to keep them at better temp co. between the four of them. This still may not work within a environment such as an amp or pre.

Cheers George
 
georgehifi

georgehifi

Member
Joined 2003
lightspeedattenuator.com
If the stage after the Lightspeed is 100k input impedance then the input impedance of the Lightspeed is between 10k-15k,
Here is the Graph that Tom Gootee plotted for the input impedance of the Lightspeed Attenuator (how I make them) for three different poweramp input impedances.
Thanks again for the graphs Tom!
http://www.diyaudio.com/forums/anal...uator-new-passive-preamp-462.html#post3015694

Cheers George
 
J

JoeyGS

Member
Joined 2007
studiostevus said:
I am using the LSA as volume control in my aikido preamp (LSA is the input stage).

What is the input impedance of the LSA, as I am selecting a suitable output capacitor size for my dac.

Thanks

---
edit; CONGRATULATIONS GEORGE!! I think we can all agree!
Click to expand...

What would be a good solution is to put an Aikido ACF between your DAC and the Aikido preamp/LSA combo. This would eliminate the impedance matching on any preceding component to your Aikido Pre/LSA.
 
You must log in or register to reply here.
Top Bottom