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Leifer  
#1 Posted : Thursday, October 6, 2022 8:31:55 PM(UTC)
Leifer

Rank: Member

Groups: Member
Joined: 10/6/2022(UTC)
Posts: 2
Location: LA,CA and MSP, MN

Just when you thought there could be no more EZ questions Dancing

I understand the basic schema for attaching a single-ended (SE) input to the EZ headphone amp......
....and I'm simplifying this for novices.


For example, one channel/one EZ board/ stock LMExxxxx chips :

External SE+ signal goes to EZ IN+ (pos).
External Ground to Ground.
EZ IN- (neg) jumpered to Ground.
EZ Out to right or left headphone jack pin.
Ground to headphone jack.
(assume a star ground for all)

The 4 Resistors....

R3 and R5 are the same value for a gain of 2 (keeping the pair R3,R5 under 5000 ohm each)
R2 = 100 ohm
R4 = 100,000 (100k) which gives the circuit a 100k input impedance.

Now my questions....

A) Level control: I have an Alps 10k (LOG/dual/stereo) potentiometer....to be added just before the EZ boards (standard volume pot configuration). Do these resistor values still make sense for typical "studio line level" signals ? (+4dB, around 1.75Vpp) (not the consumer -10 dBV)

B) If I use this as a permanent headphone amp module inside my "eurorack synth", where levels are hotter (up to 10Vpp). How do the resistors change ?
(Eurorack synth modules "typically" have a 100k input impedance, and a 100 ohm resistor at the output.)

C) If I want this headphone synth module to have an extra set of "thru" jacks, how does that affect the impedance/loading ?..... say if I just passively "mult" (split) the input ? Basically I would be tapping the synth signal to listen on headphones, as it goes elswhere in that system.


(edit: I don't want someone to just "tell me" the gain and input resistors, I'd like to learn "how and why", but some examples would be nice..... plus the simple math.)
'
.

Edited by user Friday, October 7, 2022 1:41:12 AM(UTC)  | Reason: specifics/format

Leifer  
#2 Posted : Thursday, October 13, 2022 1:53:56 PM(UTC)
Leifer

Rank: Member

Groups: Member
Joined: 10/6/2022(UTC)
Posts: 2
Location: LA,CA and MSP, MN

I tried various configurations, and my results are below.

ventus_updated_10_22 copy2.jpg (295kb) downloaded 4 time(s).

In the eurorack synth environment, I can't have 10Vpp through to the headphones (or my ears).

The 10k audio potentiometer selects an appropriate level.
But the range was not ideal.... it got too loud, too quickly.....un-useable past 12noon knob range (~50% rotation).
I added a "-10db pad" before the pot, which helped. It is still not ideal. (w/250 ohm headphones)
I also added a dc-blocking cap at the input.

If there better or alternative changes, let me know.

=R=

edit:
Using an online calculator, here is a suggested circuit output...

Headphone sensetivity @ 1mW (dBSPL) = 96
Headphone impedance ...........................250 ohms
Desired loudness (dBSPL)........................110 dB

Required power..........................................25 mW
Required voltage..........................................2.51 V RMS (7v peak-to-peak)
Required milliamps.......................................10 mA RMS
.

Edited by user Thursday, October 13, 2022 2:19:25 PM(UTC)  | Reason: format

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