I've got an electric piano (Kawai MP11 to be exact) plugged into my hi-fi amp (Yamaha 7.1 home theater amp). The amp volume is usually set at -45 dB for other inputs. For the piano I have to crank it up to -15 dB.
Is there a simple stereo preamp circuit I could build and put between the piano and amp so that I wouldn't have to mess with the amp volume?
Is there a simple stereo preamp circuit I could build and put between the piano and amp so that I wouldn't have to mess with the amp volume?
I'm scratching my head about what's the bigger mess 🙄...
Anyway, ist there no swell pedal or volume knob attached to your Kawai?
Best regards!
Anyway, ist there no swell pedal or volume knob attached to your Kawai?
Best regards!
A simple op-amp circuit from Elliott Sound Products site? I'm intrigued as to why you are adverse to messing with the volume?
Mainly because I always forget to turn it down after playing the piano, and the next person who turns the hi-fi on to listen to the radio has to change their underwear.
Okay, so you need a simple preamp with 30 dB of voltage gain.
I'm a little shocked at how huge this disparity between your signal sources is, but I'm guessing the keyboard puts out a nominally 100 mV "instrument level", while your other signal sources are putting out a few volts (CD and DVD players were notorious for this.)
A quick Google search didn't turn up anything that fit the bill, so I designed you one.
This is a very simple circuit, and it's all you need for this particular job. It uses a single NE5532 dual op-amp for both channels. I've set the voltage gain to 32 dB, and there is a volume control at the output that you can use to dial it down if it turns out to be a bit too much gain.
Frequency response covers the entire audio band, without the ridiculous excess typical of today's "audiophile" garbage. For this line level signal, noise performance should be excellent.
To keep noise and hum down, the circuit should be housed in a metal box that is connected to ground.
You need a clean +/- 15V DC power source for it, but that is a separate issue. There are a thousand schematics for a power supply like this on the 'Net, but these days I would probably just buy a couple of small, cheap, 15V switching power supplies, wire the outputs in series, and add an RC section to each power rail to filter out the remaining noise.
If you like re-purposing stuff, a lot of HP inkjet printers use a 0,+16,+32V power supply (with a flat, three-wire plug that goes to the printer.) If you can find one of these lying around the house or at a nearby thrift store, they make excellent op-amp power supplies: wire the +16V line as ground, the former 0V line is now (-16V), and the former +32V line is now +16V. Perfect. (Just add an RC filter to both +16V and -16V rails, something like 100 ohms and 220uF - 1000uF usually works well for me.)
By all means keep the volume control on this preamp at zero - as well as the volume on your Hi-Fi receiver - the first time you try it out. I'm still a bit leery of the supposedly very low output from your keyboard - if that turns out to be caused by a loose plug or something like that, and the keyboard suddenly spits out a volt of signal, this preamp circuit can spit out something over 25 volts peak-to-peak, and you do NOT want that to reach your Hi-Fi receiver input!
Edit: Your tweeters are on borrowed time if you continue playing your keyboard through your Hi-Fi receiver and speakers.
-Gnobuddy
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!Why is that? Do you assume receivers are always clipping? Is there a lot of garbage in the signal from a keyboard? What is different from listening to a piano cd?
(I am not criticising. Just trying to understand.)
(I am not criticising. Just trying to understand.)
Precisely.
Part of the issue is because the piano sits between the speakers. When I'm playing I need to crank the amp up more than if someone else was playing with me sitting on the couch. Not optimal, the room sets the constraints.
Thanks a lot. I also couldn't find anything to fit the bill by Googling.
Sounds good to me. I'll give it a go once I've sourced a suitable power supply. Thanks again.
How come?
Just for giggles, wondering if you have a parameter limiting your output voltage. http://kawaius.com/wp-content/uploads/2018/08/MP11-Owners-Manual.pdf Around page 4 there is the option to configure amplifier types. Maybe something in there for you.
It's just the bigger dynamics coming out of a musical instrument which may lead to clipping that you may not even notice. But the tweeters will 😱.
Best regards!
Part of the problem is that an (acoustic) piano is capable of tremendous loudness, which requires a correspondingly large amount of power into the speakers. If a performer turns up his/her electronic piano to even a fraction of the loudness of the real thing, it is enough to seriously endanger most Hi-Fi type speakers (specialized P.A. speakers are designed to take the abuse.)
Another part of the problem is that most electronic keyboards not only have piano sounds, but also various organ and synth sounds. Those can pump out sustained, continuous tones at any frequency on the keyboard, including well into the tweeter's frequency range. Hi-Fi tweeters can handle very little sustained power, and will usually die very quickly if you play synth or organ music through them.
I can tell you for sure that these aren't hypothetical concerns - I knew a chap who hooked up his Alesis keyboard through an Alesis power amp to his Alesis monitor loudspeakers, only to blow the tweeters within a month - and this was just from playing in his tiny mobile home (prefabricated housing in some countries.)
Those speakers cost $400 each ($800 USD for the pair) when this happened in the late 1990s, so this mistake hit his wallet pretty hard.
All the major music electronics companies make speaker systems that are designed to cope with the abuse a keyboard will hurl at them - often wedge-shaped "keyboard monitor" systems. They never use a delicate dome tweeter like a home Hi-Fi system does - they either have a compression tweeter with a horn, no tweeter at all, or a tough-as-nails piezo tweeter with a horn.
The catch is, those keyboard monitors, at least from the big manufacturers, tend to also be expensive. And you need at least two of them, since keyboards are invariably stereo output.
Perhaps Rockola (the OP) has iron self-discipline and plays at very low volume, and this could ensure the continued survival of his tweeters. But disaster is one twitch of the volume knob away.
-Gnobuddy
I understand this well. While I have negligible keyboard skills, I have a keyboard I use to study music theory, compose songs and melodies, play simple parts on my home recordings, and act as a drum machine either for jamming or recording. And I cannot find a good place in our tiny apartment to put the darn thing!
You're very welcome. 🙂
For quick testing of circuits like this, I often just wire a pair of 9V flat batteries in series. These sorts of op-amp circuits will run just fine on +/- 9V instead of +/- 15V, the only difference is there will be less input headroom, and less maximum output level.
If you like the circuit and it performs for you, you probably will want to buy/build a real power supply, because those little 9V batteries don't last all that long, and it's one more thing you have to remember to power off.
Speaking of power, I forgot to include a power-on indicator in the circuit...I suggest a high-brightness green LED, and a series resistor to set current to roughly 0.1 mA. That's usually plenty bright with today's high-brightness LEDs (anything over 5000 mcd should do.)
We should definitely investigate Printer2's link - if your keyboard has a setting to switch it's output to line level, that might solve all your problems without needing a preamp of any kind.
Another possibility to consider is to buy a small, cheap analog mixer, and use that rather than a DIY build. It might be better-looking for the living room, and may end up costing less, and offering more flexibility and features.
A quick look at the Thomann UK site (I can't read their Finnish site!) turned up several possibilities. Please double-check suitability yourself, in case I overlooked something!
1) Mackie 402 VLZ4 – Thomann UK
2) Mackie PROFX4v2 – Thomann UK
3) Allen & Heath ZED-6 – Thomann UK
4) the t.mix mix 802 – Thomann UK
-Gnobuddy
Your piano has XLR & 1/4 Jack outputs. Have You tried both ?
On the XLR just connect pin 1 - GND and pin 2 - Hot to the RCA's.
Depending on the output circuitry You may short PIN2 - Cold and GND.
On the XLR just connect pin 1 - GND and pin 2 - Hot to the RCA's.
Depending on the output circuitry You may short PIN2 - Cold and GND.
EXACTLY how have you made this connection? What output port and cable are you using? The simplest would be to use the 1/4 output jacks and use a pair of 1/4 TS to RCA cables, and if this output isn't fixed(the manual is unclear) turn up the volume control all the way of necessary.
One reason why I suggested those particular mixers is that all four have at least two XLR inputs. 🙂
The last of the four has four XLR inputs, and costs 58 Euros. It will be hard to undercut that price with a DIY build, and the DIY version will have far less capability.
For example, one could plug a microphone or two into the other two XLR inputs, and one or two people could sing along to the keyboard. Or one could plug an electro-acoustic guitar into one of the other inputs, and have keyboard, guitar, and vocals. This sort of thing can be a lot of fun!
-Gnobuddy
I don't know if Rockola is still interested in this thread, or chose to buy a small mixer and be done with it.
But just in case it's useful to anyone: I just found this small and inexpensive power supply module at Digikey: https://www.digikey.com/product-detail/en/recom-power/RAC10-15DK-277/945-3126-5-ND/7807874
The module itself is surprisingly small at around 1" x 1" x 2" (about 2.5 cm x 2.5 cm x 5 cm). It has two pins you wire to the incoming AC power from the wall outlet. You get 0V, +15V DC, and -15V DC from three pins at the other end, at up to 340 mA. Simple as that. The price is under $12 (USD), or under $16 (CAD).
I looked at the spec sheet. Internal switching is at 100 kHz, and maximum power rail ripple (probably at the full 340 mA current draw) is 1% of 15 volts, or 150 mV.
150 mV of ripple is too much for sensitive audio electronics, but the high 100 kHz frequency makes it very easy to filter out the noise. If you put a simple RC filter - say 10 ohms series resistance and a 100 uF electrolytic in parallel with a 100 nF ceramic cap wired across each DC supply rail - that should reduce the ripple down to well below 1 mV.
This looks like a small, light, convenient, and inexpensive way to power most typical op-amp projects that require plus and minus 15 volt rails.
-Gnobuddy
> this small and inexpensive power supply module
For your next trick....
This thing is also rated for DC input! To 430VDC! Which fits the smaller tube guitar amps. Interesting way to add opamp-supply to a tube amp without a lot of parts or tapping into the line-wiring.
For your next trick....
This thing is also rated for DC input! To 430VDC! Which fits the smaller tube guitar amps. Interesting way to add opamp-supply to a tube amp without a lot of parts or tapping into the line-wiring.
I've seen a few schematics of guitar amps from the 1980s that did this (op-amps mixed in with tubes.)
Unfortunately, since that time, guitarists seem to have divided into two camps, and nowadays most in the tube amp camp have a superstitious hatred of op-amps, and will not touch a guitar amp that uses them.
Personally, I have concerns about the limited headroom (and abrupt clipping) of op-amps compared to tubes running on several hundred volts. But I can think of specific applications where this might not be an issue, such as providing a speaker-emulated instrument-level signal to feed directly to the sound reinforcement system.
-Gnobuddy
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