Converting an old Hi-Fi to use as a PA

stu

Okay so some really cool dude donated to me an old home made Hi-Fi amp and some speakers. The speakers work but the amp doesn't, but he said it can be fixed.
And since there are so many clever guys on the forum I thought this would be the perfect place to ask.

Can I convert this lot to use as a PA system for my band?
I dont know very much about amps but could the Red and White plugs be changed to XLR so a microphone could be plugged straight in? Or could the amp be made into a power amp? Ideally I would like to have more then one microphone plugged in at a time so I would think converting it (if possible) into a power amp would be best.

Any other thoughts?

costafonix

hmmmm :-\ not sure how well that would work, what is the wattage of the speakers/amp etc? you'd probably do better to save the money you'd use to repair the amp and put it towards a proper PA system, ideally you'd need a mixer as well to plug your mikes in to get a decent mix for your band.

AlanRatcliffe

Not really suitable. Hi-fi amps don't have proper mic inputs and only input a maximum or two channels at one without mixing them (which helps spread the load over both amp channels and speakers), so you would need to use at least mic preamps or ideally a mixer with mic pres built in. Even then, they are rarely made to take any kind of punishment, which is crucial for PA.

Hi-fi speakers are the real weak point though - PA often has to push most of it's power in a narrow frequency spectrum and hi-fi voice coils tend to overheat and burn out or their cones overextrude causing damage in those conditions.

guitarboy2828

Alan Ratcliffe wrote:
Hi-fi speakers are the real weak point though - PA often has to push most of it's power in a narrow frequency spectrum and hi-fi voice coils tend to overheat and burn out or their cones overextrude causing damage in those conditions.

+1 to this. Hi-fi speakers won't work, unless you're have very quiet band practices, they won't keep up. It's a pity because getting hifi equipment is a lot easier than getting PA equipment ?

stu

Thanks for the help guys. Its quite disappointing that its not going to work out.
And I see that a small PA system is like 300 watts, the amp I have is 60 watts and I dont know what the speakers are (I think 80 watts)

But that brought up a question for me. When a 60 watt Hi-Fi gets turned up its freaking loud. Like the kind of loud that you cant hear anything else. So then why is 60 or 80 watts to soft to use for a PA? Because if 60 watts can deliver that kind of power then surly 300 watts could kill someone?

Wizard

Silly question.
This thread has proved that a hi-fi amp doesn't work well as a PA.
How well does a PA work as a hi-fi amp?

AlanRatcliffe

stu wrote: Thanks for the help guys. Its quite disappointing that its not going to work out.
And I see that a small PA system is like 300 watts, the amp I have is 60 watts and I dont know what the speakers are (I think 80 watts)

150W are the small entry level systems these days.

When a 60 watt Hi-Fi gets turned up its freaking loud. Like the kind of loud that you cant hear anything else. So then why is 60 or 80 watts to soft to use for a PA? Because if 60 watts can deliver that kind of power then surly 300 watts could kill someone?

It's complicated. Wattage has little to do with volume and you need roughly 10X the wattage to double the volume. On the other hand, your Hi-fi amp has two channels, each at 60W so it's comparable with a 600W mono amp/speaker. But then again, the hi-fi speakers are a lot less efficient, so you probably lose a fair amount of volume compared to a more efficient PA speaker. As I said... it gets complicated. ?

Wizard wrote: This thread has proved that a hi-fi amp doesn't work well as a PA.
How well does a PA work as a hi-fi amp?

Also doesn't fare well - horses for courses. PA is made for volume and coverage and both come at the expense of fidelity.

guitarboy2828

Correct me if i'm wrong Alan, but doesn't frequency play a big role in this too.

A CD player/tuner into an amp is a completely different sound frequency, as opposed to a raw vocals or bass guitar. Correct?

I had a very nice Hifi system that I wanted to use as a bass amp, my dad (who is a technician) said won't work because it's not meant to handle the raw frequency of a bass guitar.

AlanRatcliffe

guitarboy2828 wrote: A CD player/tuner into an amp is a completely different sound frequency, as opposed to a raw vocals or bass guitar. Correct?

The frequencies are the same (20Hz to 20KHz), but it's the distribution of power within those frequencies. With hi-fi, the power will be spread over the whole range, while plugging a bass guitar in (or mic) and cranking it means that all the power goes into only a narrow part of that spectrum and the speaker must extrude a lot further that it is designed to do.

Similarly the amp is not designed to take the abuse. With hi-fi the power is largely used for headroom - most of the music is at lower levels and the amp merely needs to handle occasional momentary peak without distortion and the average power level is quite low. With PA, you are usually operating in the upper limits of power with less headroom for peaks, which puts a lot more continual stress on the amp. So PA amps are designed (or overdesigned) to cope with this without overheating or generally doing antisocial things like blowing up in the middle of a gig. Of course, they are also designed to take the serious physical abuse which happens on a daily basis with gigging.

TomCat

Another big difference here is that a HiFi reproduces an already recorded source so the only sound heard is the reproduced sound.
In a PA the sound is being produced or amplified along with the unamplified sound of the source. In a band room the sources will be vocal microphones, keyboards etc that have to firstly be turned up above the unreinforced sound connected to the PA and secondly overcome other sounds in the room such as a drumkit, guitar and Bass amps etc.

The general rule is that about 300w of power is required to compete with a drumkit so 2 x 150W PA would be considered a minimum.

Hifi loudspeakers as mentioned are very inefficient, usually about 89dB@1W@1m whereas your run of the mill PA loudspeaker is sitting at 98dB. The HiFi amplifier would need to have 8 times more power with HiFi loudspeakers than the PA amplifier with it's loudspeakers to achieve the same level. i.e a 300W PA is equivalent to 2400W HiFi.

One last thing......

The dynamics of a recording is a lot smaller than the dynamics of live reinforcement as compression has been applied to the recording to reduce peaks and therefore make the recording sound louder with less power.

BTW all that has been said above regarding frequency/ spectrum power-bandwidth etc is also valid.

stu

So if I understand correctly there is a lot more load and stress that is put onto the speakers and amp of a Hi-Fi if you where to run a mic into it?

Now this cant be the same if you where to play a CD with someone singing on it.
So I could prerecord the vocals and play it loud through the hi-fi at band pratice rather then having live vocals. I just had that idea today.

I know I should just give up on this idea and get a poper PA ,but I hate giving up ?

guitarboy2828

Oh wow, this is all very interesting.. The thing one learns! ?

guitarboy2828

stu wrote: So if I understand correctly there is a lot more load and stress that is put onto the speakers and amp of a Hi-Fi if you where to run a mic into it?

Now this cant be the same if you where to play a CD with someone singing on it.
So I could prerecord the vocals and play it loud through the hi-fi at band pratice rather then having live vocals. I just had that idea today.

I know I should just give up on this idea and get a poper PA ,but I hate giving up ?

That is interesting.. I eagerly await the response to this.. Because really, that is like a "cd"... Isn't it? haha..

FenderBender

stu wrote: So if I understand correctly there is a lot more load and stress that is put onto the speakers and amp of a Hi-Fi if you where to run a mic into it?

Now this cant be the same if you where to play a CD with someone singing on it.
So I could prerecord the vocals and play it loud through the hi-fi at band pratice rather then having live vocals. I just had that idea today.

I know I should just give up on this idea and get a poper PA ,but I hate giving up ?

guitarboy2828 wrote: That is interesting.. I eagerly await the response to this.. Because really, that is like a "cd"... Isn't it? haha..

This is a real die hard thread ?

Yes, if your vocal is recorded properly, it is essentially the same as CD, because the raw mic signal would have been compressed and EQd by the sound engineer and recording equipment.
If you are going to go ahead with this, remember that the line in on your hi-fi amp is expecting a consumer audio signal that is a lot lower than professional audio, so depending on your recording and playback equipment you might need to set that volume pot lower than usual to achieve the maximum output of the amp. Anything higher and your system will clip badly.

Of course you won't be able to hear it over the drums anyway..... ?