How to drive Logitech H151 headset speakers? - Page 1
Forum section: electronics / beginners

author: Winkhaus
author_rank: Contributor
timestamp: April 27, 2025, 01:26:49 am
content: Hello,I have a basic analog (not USB) Logitech H151 headset, and I'm trying to figure out how to play a waveform through the speakers.The headset has an inline volume control, and when the volume control is all the way at the (-) setting, I measure the DC resistance of the speakers as 4Ω each (measured at the TRRS connector), and when it's all the way at the (+) setting, I measure 26Ω. This is opposite from what I expected, but I'm also concerned about the low impedance somehow putting too high of a load on the amplifier with the volume control on low, even though the volume would then be low or zero.Can someone tell me how this works and what op-amp or power amp I should use here? I believe the speaker wattage ratings are roughly 60mW each.Thanks,David

author: amyk
author_rank: Super Contributor
timestamp: April 28, 2025, 07:42:18 pm
content: What amp are you using? 4 ohms is not an uncommon output impedance for audio amp power to be specified for. The one in any computer audio chip will drive it just fine.

author: Winkhaus
author_rank: Contributor
timestamp: April 28, 2025, 09:27:57 pm
content: Well I was thinking of using the SSM2250, but the datasheet says: "The SSM2250 features an audio amplifier capable of delivering 1.5 W of low distortion power into a mono 4 Ω bridge-tied load (BTL) or 2 × 90 mW into stereo 32 Ω single-ended load (SE) headphones." So would I then need two of them, with a BTL configuration for each speaker?I also looked at the LM386, but the fixed gain of 20 is a lot more than I need, so I guess I would have to voltage divide the input, then re-amplify. Seems like there should be an easier way.And I still don't understand the situation with the inline volume control. I'm thinking the speakers themselves are actually 26Ω and the 4Ω reading is because there is a potentiometer in parallel with the speaker connections. Why would they do this? Wouldn't it be less of a load on the amplifier at low volume if the potentiometer were in series?

author: amyk
author_rank: Super Contributor
timestamp: April 29, 2025, 12:46:53 am
content: That's the maximum capabilities of that amp, but keep in mind that for headphones, 1.5W is a LOT of power, an absolutely deafening amount. I found that the specifications for your particular model show a sensitivity of 122dB/mW, i.e. with only 1mW of power input, it will produce 122dB of sound pressure. 122dB is well into the threshold of pain. 1.5W is 1500 times that. You will not need to turn the volume up to anywhere near the maximum.

author: Winkhaus
author_rank: Contributor
timestamp: April 29, 2025, 03:52:36 am
content: Yes, I understand, as mentioned, I believe the max power ratings of the speakers are about 60mW each. I wasn't planning to try to crank them up to 1.5W. My concern was about the impedance being so low, with the inline volume control turned all the way down, that it could risk damaging an amp.Re: sensitivity, are you sure it's 122db per mW, not per Watt? Hopefully when/if I get this working I can investigate further. I did a little more research and I'm going to try an LM4864 for the power amp.

author: amyk
author_rank: Super Contributor
timestamp: April 29, 2025, 10:59:02 am
content: If the amp is rated to output up to 1.5W into a 4-ohm load, it's not going to be bothered outputting a few hundred uW (at most) into the same load. Yes, headphones are rated in dB/mW sensitivity (and rarely dB/mV), not dB/W.