70W - 120W Amplifiers

[Gareth Young]

I am looking to replace a broken amplifier in a pub which creates unwanted noise on the input stage of one channel. The current amp is a hi-fi type receiver/amp unit which provides 70W @ 8 Ohms RMS for <1% THD at 1kHz. I am looking to replace it with a conventional amplifier that can drive a similar load or maybe slightly more (70 - 120W @ 8 Ohms) but don't have much experience in this end of the audio market. Upper limit on price is £250 and units I've seen so far that might be able to do the job are:

 

Gemini: P07, X-01, X-02, GPA2000, GPA1000

Samson: Servo 300

Alesis: RA300

Carver: ZR500

 

Does anyone have any experience with these products and how they perform? Or does anyone think they know a better type of amp for the job I haven't found yet? The landlady seems to like the music loud so I would expect the amp to be able of working quite hard continuously without thermal or other failures.

 

For those who care about the details, its a strange set up with TOA F-5 cabinets around the pub with 4 or 5 cabs on each amp channel, connected in parallel. What is strange is that although they claim to be 10W 8 Ohm units, upon metering them on the back they seem to be about 30-35 Ohms per cabinet. The drivers all still work however, and conveniently when they are connected in parallel together the impedance drops to a sensible 10 Ohms or so, which is what the amplifier sees. How or why the boxes are at 33 Ohms is a mystery to me, although whoever installed it seems to have done it deliberately to obtain a total impedance of around 8 Ohms per channel for the benefit of the amp. This is a point of curiosity for me, ultimately all I want to know is how potential replacement amps compare. However if anyone knows whats going off with the boxes it would be interesting.

[Simon Lewis]

Carver made a reasonable name for themselves in the Pro Audio world, but their amps weren't quite as good as the linear PSU types they sought to replace. The ones you quote are their new wave - I haven't had a chance to try them.

 

The Samson units are suprisingly good. I've used many Servo 550s in churchj installs where fan noise is an issue. Unfortunately, they've ditched that one and replaced it witha fan cooled version! I'm not sure whether the 300 has been upgraded too.

 

No hands on with Alesis or Gemini, but they are aimed at the MI / Dsico market segment.

 

Simon

[bruce]

Accordign to the spec on the TOA web page, it appears that this speaker (which is indeed 10W), can either run as an 8 ohm unit, or with a 100V input.

 

The fact that they're wired in parallel suggests they're wired in a 100V config. So you may need a 100V amp rather than an 8 ohm one...

 

Check how the speakers are wired up - you apparently need to change an internal link to swicth between 8 ohm and 100V mode.

 

See http://www.toaelectronics.com/disc/spec_sheets/f-5.pdf

 

Bruce.

[Gareth Young]

Simon - Thanks for the advice, fan cooling noise is not going to be an issue for this situation as it is intended for use in a pub so is unlikely to be heard - if it means the Samson amps cool more effectively, then that is actually a bonus for my situation given that they may be driven quite hard.

 

Bruce - I did check the TOA datasheet beforehand and that is why I posted the question of why the drivers measure ~30 Ohm resistance. The F-5 can be operated as a 100V serial line system, but the pub has no step up transformer and was run off a hi-fi type 70W amp/receiver unit, which certainly does not have a 100V output. Also, the F-5 datasheet states a 500/1000 Ohm impedance when the internal step down transformer is connected for use in a constant voltage system and that is miles away from the ~30 Ohms measured. Hence the confusion - the value I measure does not match with either the passive cabinet value or the value for constant Voltage use. I have not yet been able to look in the back of one of the cabinets, but my working hypothesis is that the PA installer has placed resistors in series inside the boxes to achieve the desired total impedance or has replaced the drivers in the cabinet to some unusual 30 Ohm impedance type. However, both ideas seem quite wrong to me, but then it is a small pub install, so anything is possible.

[mackerr]

The F-5 can be operated as a 100V serial line system, but the pub has no step up transformer and was run off a hi-fi type 70W amp/receiver unit, which certainly does not have a 100V output.

A 100V doesn't really have a 100V output either. Audio ranges from 0 volts during silence to whatever the maximum output of the amp is at clipping.

Also, the F-5 datasheet states a 500/1000 Ohm impedance when the internal step down transformer is connected for use in a constant voltage system and that is miles away from the ~30 Ohms measured. Hence the confusion - the value I measure does not match with either the passive cabinet value or the value for constant Voltage use.

While the resistance you measure will be less than the impedence unless you are using an impedence meter, you are right, it seems way low. Are you using an ohmmeter, or an impedence meter?

I have not yet been able to look in the back of one of the cabinets, but my working hypothesis is that the PA installer has placed resistors in series inside the boxes to achieve the desired total impedance or has replaced the drivers in the cabinet to some unusual 30 Ohm impedance type. However, both ideas seem quite wrong to me, but then it is a small pub install, so anything is possible.

I think a look inside is definately in order. I know that when measureing the DC resistance of a voice coil, you expect the DC resistance to be 25-30 percent lower than the average impedence. I do not know how much the value will drop when you are measuring a small transformer where the secondary winding is across that voice coil.

 

Mac

[Dweeks]

If possible, why not bridge the outputs to the channel thats working, bit of clever rewiring and you're sorted....