a couple of questions about a lm1875 guitar amp

[neebster]

I am curious regarding power output of the LM1875. On the datasheet, the graph for input voltage/output power is rated at 8ohms. I am curious as to the power curve at 4 ohms.

More specifically, I am looking to use a 15 watt guitar amp speaker, and therefore want to limit the amp to 15 watts. So I am trying to find out what transformer secondary voltage I need to achieve 15 watts with a standard chipamp.com LM1875 kit.

Also, does anyone have any input on whether to try to match the output impedance of the amp to that of the speaker? I've read about this, but am not sure what to think.

[Leolo]

One big difference between a guitar amp and a hi-fi amp is that the guitar amp is designed so that it will survive when driven at high levels for a long time. Distortion is the friend of guitarists. Conversely, the design of hi-fi amps assume that any distortion will be temporary.

Practicaly, this means a large transformer (high VA, but low V because you want low wattage) and lots of caps in the PSU. And a large well ventilated heat sink.

There are probably other considerations I haven't thought of.

[i2k92]

You'll need a guitar preamp & speaker simulator in front of it, to make it sounds like guitar amp not like hifi amp. You can use preamp with simulators like Line6 PODs or Sansamp GT2 (you can find the pcb here : tonepad -- a resource for d.i.y. music projects). GT2 is the best imho.

[AndrewT]

Move your inquiry to musical instruments.
You are more likely to get appropriate advice for a musical instrument.

[neebster]

I was planning on something like:

PAiA stack-in-a-box >> one channel of an Oatley K295 stereo tube buffer >> passive tone circuit >> second channel of Oatley K295 >> LM1875 from chipamp >> Weber Signature 6" 4 ohm guitar speaker

So, my inquiry is regarding:

A) What transformer voltage do I need with a 4 ohm load to get 15 watts with a standard chipamp LM1875 kit?
and
B) Do I need to do some type of voodoo to get the output impedance of the chipamp to be 4 ohms, instead of ~0 ohms?



The PAiA kit will take care of making it "sound" like a guitar amp, as it has some overdrive and also cabinet emulation.

As for the PSU:
A) I was thinking that first of all, I would play through the preamp "clean", then listen for any distortion with the amp cranked, and adjust the buffering preamp gain to get the levels right.
B) I will use a 60-80va transformer, with only 1 channel at 15w it should handle it. That's 4+ x as much power rating as I would use.

Do I need any other considerations with the PSU?

[danielwritesbac]

Here is the requested voodoo on output impedance. Just turn the dial.
Lenard Audio - Education - Guitar Valve Amps.

That works for guitar speakers, tube era sealed box speakers, lowther, fostex and similar products.

The easy chip amplifier for 4 ohms is Non-Inverting TDA7294.
convenient single chip boards can be found--use one that's simple.
Add 330uF at both of the pre power pins, but otherwise the datasheet example is functional.
Antec has some nice 22,22 transformers. 144va is minimum.

Include full dynamic range for power calculations with guitar amp (double power adds only 3db to speaker). You'll need an extremely sturdy and powerful chip for live music! You will also need a chip with relatively benign clipping behavior--No spike! Some power supply options alter clipping behavior as well. See Rod Elliot and Lenard Audio.

[Minion]

Originally Posted by neebster

I was planning on something like:

PAiA stack-in-a-box >> one channel of an Oatley K295 stereo tube buffer >> passive tone circuit >> second channel of Oatley K295 >> LM1875 from chipamp >> Weber Signature 6" 4 ohm guitar speaker

So, my inquiry is regarding:

B) Do I need to do some type of voodoo to get the output impedance of the chipamp to be 4 ohms, instead of ~0 ohms?



The PAiA kit will take care of making it "sound" like a guitar amp, as it has some overdrive and also cabinet emulation.

I have Built a PAiA Stack in the box and it is good a good little preamp but it totally lacks in the Tone shaping department , I also tried a 3 band Passive Tone stack on the output but it sounded like *** , What I ended up doing was useing a 15 band EQ on the output and then it sounded great and was easilly able get allmost any tone I wanted out of it ......

Here"s a Pic ....


I don"t think you understand Impedances , while the amp it"s self has a output impedance under 1 ohm that doesn"t mean you can drive a 1 ohm speaker , if you tried you would dammage the amp , the amp wants to see a load of several times it"s output impedance and the Higher the load the easier it is for the amp to drive it .......

What will determine the lowest load you can safely drive with the amp is it"s power supply voltage , the Lower the Voltage the lower the load it can safely drive , that is why the data sheet gives specs based on supply voltage and speaker load ......

If you use +/-20v DC rails it will happliy drive 4 ohms or higher load .......

[danielwritesbac]

Yes, a 28vct (14,0,14) transformer for LM1875 with a 4 ohm speaker.
14vac * 1.45 = ~20vdc

Nearest matches include this:
Link to. . . Example for Toroid
and this:
Link to. . . Example for EI Core


Use input cap, use nfb cap. You don't want DC offset when the amp is already working hard.
Heatsink very, very well.

[neebster]

14vac. Perfect.
I have a very large heatsink I intend to use, taken from an old broken amp.

I was concerned about the tone of the paia unit. I may go with five bands.
How about 5-Band Graphic Equaliser Kit (1044)

[danielwritesbac]

That equalizer has coverage for the full range of a guitar speaker. It should work.

The Lenard-Elliot variable current drive dial might be more useful for controlling the tone of the speaker though.

LM1875 at 40VDC to a 4 ohm speaker has at most 10 watts capacity for live sound dynamics. So, are you using a compressor? Do you intend for a great deal of clipping every time you play very loud? Or do you need to select a more powerful chip that can run at Higher Voltage (less clipping), such as this: 50W Mono Hi-Fi Audio Power Amplifier Module TDA7294 (3106) conveniently from the same vendor as your equalizer. Both it and the LM1875 are capable of running Lenard Audio Variable Current Drive so as to adjust output impedance via convenient dial. However, TDA7294 is far more durable for your application. Do you want your project to last longer than ~6 months?