Hey everyone, I'm getting the supplies together to build a power amp using the LM3886. I'd like to set it up so that it can run two sets of speakers, either individually or simultaneously. From reading, it seems that toggling the output signal can be unsafe, so instead I'm thinking of building two separate stereo LM3886 modules (using two these https://chipamp.com/product/non-inverting-lm3886-stereo-amplifier-kit/), and having a 3-position rotary switch redirect the RCA input signal to one set or the other, or both.
A few questions:
1. Would this just require wiring the RCA input signal to the rotary switch, with two separate outputs that go to each LM3886 board? (and grounding the the RCA jacks to the central chassis ground)
2. I'd like to have it so that a single power switch turns on both amp modules at the same time. Is this as simple as connecting both transformers to single power module and/or power toggle switch? Or, conversely, would it be possible to use a single transformer to power both amp modules?
3. If I add a volume pot to the whole mix, can I just do this in between the RCA inputs and the rotary switch? (thus, having one switch that controls both volumes)
3. Is this a terrible idea? Or is there a better way to do this that doesn't require two separate amp modules?
Thanks!
A few questions:
1. Would this just require wiring the RCA input signal to the rotary switch, with two separate outputs that go to each LM3886 board? (and grounding the the RCA jacks to the central chassis ground)
2. I'd like to have it so that a single power switch turns on both amp modules at the same time. Is this as simple as connecting both transformers to single power module and/or power toggle switch? Or, conversely, would it be possible to use a single transformer to power both amp modules?
3. If I add a volume pot to the whole mix, can I just do this in between the RCA inputs and the rotary switch? (thus, having one switch that controls both volumes)
3. Is this a terrible idea? Or is there a better way to do this that doesn't require two separate amp modules?
Thanks!
It's really ok to just use a switch at the hot amp outputs to select which speakers.
Most commercial amplifiers do this. You'd need a single DPDT switch of an adequate
current rating. All 4 of the speaker negatives are just directly wired to the amplifier's common.
BlndSp2ll said he wants to be able to run either pair separately or simultaneously. I believe an A or B switch can be made, for most amps, with one DPDT. I've been told there is the potential that some amps won't like combined -, or ground, wires.
However I think the option for A+B requires a slightly more complicated setup to keep impedance from being reduced.
If we forget the switch, for a moment, and just consider connecting two pairs of 8ohm speakers to the amps binding posts wouldn't the impedance become 4ohms?. So, shouldn't the switch be wired with A and B in series, or have some resistors, to keep the impedance at 8ohms?
i'm a newbie at this so I may be all wrong. However I've attached a schematic I drew that I think would keep the impedance at 8ohms. It's not completed but shows how I think the speaker wiring has to be.
However I think the option for A+B requires a slightly more complicated setup to keep impedance from being reduced.
If we forget the switch, for a moment, and just consider connecting two pairs of 8ohm speakers to the amps binding posts wouldn't the impedance become 4ohms?. So, shouldn't the switch be wired with A and B in series, or have some resistors, to keep the impedance at 8ohms?
i'm a newbie at this so I may be all wrong. However I've attached a schematic I drew that I think would keep the impedance at 8ohms. It's not completed but shows how I think the speaker wiring has to be.
Attachments
switching the output between two sets of 8ohms speakers to allow ONLY one stereo pair to play at any time is easy.
adding the function of playing both sets of speakers at the same time is very much more onerous. I would not even try.
Using two sets of amplifiers each driving a single 8ohms speaker is a workable solution. This requires the switch to be moved to the inputs instead of the outputs.
But the non selected input must not be left open circuit. Any amplifier will try to amplify any signal including interference when the input is left open circuit. Inverting amplifiers are even worse in this situation !
adding the function of playing both sets of speakers at the same time is very much more onerous. I would not even try.
Using two sets of amplifiers each driving a single 8ohms speaker is a workable solution. This requires the switch to be moved to the inputs instead of the outputs.
But the non selected input must not be left open circuit. Any amplifier will try to amplify any signal including interference when the input is left open circuit. Inverting amplifiers are even worse in this situation !
like..have 2 switches, if one of them is closed than it makes speaker A connected, and when the other is allso closed, both of them are connected in parallel.
and if only the other is closed then only B speakers are connected.
not hard.
and if only the other is closed then only B speakers are connected.
not hard.
But now you risk blowing up the chipamp because you are running two sets of speakers in parallel !
not really.
we did not discuss if the amplifer was rated for low impedance loads or not.
even if not, i can safely connect a 1 ohm car sub to say.. an lm3886 chip, just has to make sure i don't crank it up to full blast. we just have to make sure we do not exceed the current that the chip can safely provide.
if we are that fuzzy, with some relay logic we can quite simply enable an L-pad at the input of the chipamp as a crude solution, then you can turn your volume anywhere you want it to.
but i was assuming the chip can take a 8||8 ohm rated load.
we did not discuss if the amplifer was rated for low impedance loads or not.
even if not, i can safely connect a 1 ohm car sub to say.. an lm3886 chip, just has to make sure i don't crank it up to full blast. we just have to make sure we do not exceed the current that the chip can safely provide.
if we are that fuzzy, with some relay logic we can quite simply enable an L-pad at the input of the chipamp as a crude solution, then you can turn your volume anywhere you want it to.
but i was assuming the chip can take a 8||8 ohm rated load.
That was the point of my post with the sketch. I didn't think running two sets in parallel was a good thing. Very few speakers are 8 ohms over their whole frequency range. I have two sets of speakers and the mfg. specs. are 8 ohms and min. of 3 ohms. Those speakers in parallel would be 4 ohms to 1.5 ohms.BAD
That spec suggests to me they are 4 to 8ohms speakers.
I would guess they are using a pair of 4ohms bass/mid drivers in parallel to give a low frequency equivalent to a 4ohms speaker.
If that guess is correct the power amplifier should be designed for a 4ohms speaker, because the majority of the frequency range is defined by the parallel pair of 8ohms drivers.
There seem to be a number of, A, B, A+B, switches for sale on sites like Parts express. Some ads say they are impedance matching. For example two sets of 8 ohm speakers playing in the A+B mode will present an 8 ohm impedance to the amp. Maybe he should buy one if building one is difficult or risky.
I did think my schematic would work. I just haven't figured out how to connect it to a switch. Gotta check youtube.
I still have my Nakamichi RS-730 A,B,A+B switch but don't know if it is impedance matching. It's gotta be about 30 years old and I can't find any info on it. Is there a way to test it with a mutli meter?
Thanks,
henry
I did think my schematic would work. I just haven't figured out how to connect it to a switch. Gotta check youtube.
I still have my Nakamichi RS-730 A,B,A+B switch but don't know if it is impedance matching. It's gotta be about 30 years old and I can't find any info on it. Is there a way to test it with a mutli meter?
Thanks,
henry
I'll be using a Parallel86 with those speakers so it should be fine. My Modulus86 is also rated for 4 ohms so it would work. I listen to jazz and classical, no head bangin rock.
if those speakers are 4 to 8ohms and your amplifier is rated for 4ohms duty, then you cannot use two speakers in parallel.
Sorry for the confusion. I didn't mean it would be fine to run two sets in parallel. In fact in one of my earlier post I said that would be BAD.
Anyone know how I can test my Nakamichi to determine if, in the A+B mode, the speakers in parallel?
Anyone know how I can test my Nakamichi to determine if, in the A+B mode, the speakers in parallel?
This is the connection layout of the switch box. I think connections are in parallel. Is that correct?
speaker A R+ R- L- L+ speaker B R+ R- L- L+
All of the following is with the switch in the A+B position
Spkr A R+ has continuity with Spkr B R+
Spkr A R- has continuity with Spkr B R-
Spkr A L- has continuity with Spkr B L-
Spkr A L+ has continuity with Spkr B L+
speaker A R+ R- L- L+ speaker B R+ R- L- L+
All of the following is with the switch in the A+B position
Spkr A R+ has continuity with Spkr B R+
Spkr A R- has continuity with Spkr B R-
Spkr A L- has continuity with Spkr B L-
Spkr A L+ has continuity with Spkr B L+
aure it has continuity, question is what resistance it has.
if i'm not mistaken in position a+b there can be 2 outcomes.
1, its a dead short, so it connects a+b parallel to the amplifier.
2, it has some ressitors inside to get a+b parallel up to about 8 oh ism nominal.
if i'm not mistaken in position a+b there can be 2 outcomes.
1, its a dead short, so it connects a+b parallel to the amplifier.
2, it has some ressitors inside to get a+b parallel up to about 8 oh ism nominal.
A || B might work if both speakers are 8 Ω and don't have extreme dips in their impedance. It would require some analysis to ensure that the speakers meet this requirement. I would NOT recommend running two sets of speakers in parallel from one amp as a general solution.
I much prefer Andrew's solution: One power amp per speaker, switching on the input. Just make sure to ground the unused amp inputs. If the inputs are left open, you're likely to get all sorts of noise on the output. One could get fancy and implement remote power on/off of the power amps as well so the amps aren't on unless they need to be. One could put a separate volume control or attenuator pad on the amps to level match (account of differences in speaker efficiency and room gain).
Tom
I much prefer Andrew's solution: One power amp per speaker, switching on the input. Just make sure to ground the unused amp inputs. If the inputs are left open, you're likely to get all sorts of noise on the output. One could get fancy and implement remote power on/off of the power amps as well so the amps aren't on unless they need to be. One could put a separate volume control or attenuator pad on the amps to level match (account of differences in speaker efficiency and room gain).
Tom
- Status
- Not open for further replies.