Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

[Jim Thorn]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

That differential connection of the 70V transformer makes it act like a surround speaker, not a center speaker. If every source is mono, and is only connected to either a left input or a right input (but not both), it will sum as you expect. Any chance the mic's output is connected via y-cable to both left and right inputs? That would cause significant cancellation of the mic's signal at the transformer.

Jim Thorn

Hi Brad,

I'm sorry I misled you. The amp is a: Sherwood RX-4109 105 Watt Stereo Receiver. I thought my amp was the same as the one at the Park Clubhouse. The 4109 has a stereo phone input.

The wireless mic receiver does Not have a line output. Here's the info on the wireless mic system.
http://www.musiciansfriend.com/pro-audio/nady-dkw-3-handheld-wireless-system/275160000533000?source=3WWRWXGP&gclid=CKe3hbPxtLoCFUdk7AodI3MAng&kwid=productads-plaid^57307399987-sku^275160000533000@ADL4MF-adType^PLA-device^c-adid^30423866787#productDetail

The connection of the amplifier output to the Xfmr is:

The the two leads of the 8 ohm winding of the Xfmr is on the Hot Right and Left amplifier connectors. This allows either of both of the stereo inputs to be used and will be sent to all speakers. This is an old HiFi trick to connect a Center speaker as well as the left and right speakers to eliminate the ping-pong effect with some music. One stereo line input can be used for both a CD player's Left output and a DVD player's Left output. Both players would not be used at the same time, so it provides additional inputs.

Haven't had time to try the HPF yet.

Bill

 

[John Roberts]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

Hi Brad,


The connection of the amplifier output to the Xfmr is:

The the two leads of the 8 ohm winding of the Xfmr is on the Hot Right and Left amplifier connectors. This allows either of both of the stereo inputs to be used and will be sent to all speakers. This is an old HiFi trick to connect a Center speaker as well as the left and right speakers to eliminate the ping-pong effect with some music. One stereo line input can be used for both a CD player's Left output and a DVD player's Left output. Both players would not be used at the same time, so it provides additional inputs.

Haven't had time to try the HPF yet.

Bill

A subtle issue with that L-R connection, it will work the way you say, but the amp may not be designed to be operated like that.

For a full strength L or R signal the opposite channel of the power amp will be providing the 0V synthetic ground. In effect that opposite synthetic ground is sinking the max signal current while holding the voltage at 0V. For a linear amplifier output stage this is the worst case for power dissipation. Max current times full PS voltage.

In the old hifi trick, this difference L-R signal for stereo program does not contain much energy. But banging both sides with different full strength mono signals will make those opposite amp channels do some work.

The may be TMI for the OP, just thought I 'd let you know why it blew up, if it does. Of course it may just get hot and keep on truckin.

JR

 

[Craig Hauber]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

Those crappy "Sherwood" receivers can barely tolerate 4-ohm stereo loads. A local jukebox vendor was using them for extension speakers and just about all of them have been replaced by me with "real" gear. They would go into protect mode with a good heavy bass note at not full volume levels -and their protect method was simply powering-off so the barkeep would always have to run to back room to turn it back on. After a few weeks of this they would usually fail permanently.
I simply can't imagine one putting up with the abuse the OP is describing -especially after JR's posted reply.

I also can't believe this thread has gone to 22 posts without even minor chastisement and some recommended replacements.
(even the cheapo ubiquitous Pyramid 100w "PA amp" you find in the back of VFW and moose halls nationwide costs about the same, can outperform it by a long shot and last through decades of abuse -and interestingly enough I think it still comes with a real mono "phono" input

A subtle issue with that L-R connection, it will work the way you say, but the amp may not be designed to be operated like that.

For a full strength L or R signal the opposite channel of the power amp will be providing the 0V synthetic ground. In effect that opposite synthetic ground is sinking the max signal current while holding the voltage at 0V. For a linear amplifier output stage this is the worst case for power dissipation. Max current times full PS voltage.

In the old hifi trick, this difference L-R signal for stereo program does not contain much energy. But banging both sides with different full strength mono signals will make those opposite amp channels do some work.

The may be TMI for the OP, just thought I 'd let you know why it blew up, if it does. Of course it may just get hot and keep on truckin.

JR

 

[Bill Sommer]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

Hi Jim,

No, the Mic receiver is Only connected to the left channel. The right channel could be used in lieu of the left, but I only use the right channel if there is a need for two mics.

Thanks,

Bill

 

[Jim Thorn]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

Bill,

I get the impression that this is a case where you'd rather invest effort than money. If you have an unused lo-Z to hi-Z transfromer sitting around, you could try putting it between the wireless receiver's output and one of the Sherwood's aux inputs, in place of the phone preamp.

Most of the ones I've seen have a turns ratio between 6:1 and 10:1, which would give your mic signal a voltage increase of 15 to 20 dB, without the monstrous RIAA bass boost. I suspect you could then use the output pot on the wireless receiver to set a usable level.

I almost hate to admit it, but I have done such a thing using a 25-volt speaker transformer (!) for an after-hour emergency repair to a school stadium system. 1/4" hi-Z mic connector was wire-nutted to the 8-ohm winding, and the 1W/25V winding was wire-nutted to amp's aux input. I needed to bring a 30 mV signal up to 300 mV. When it's used as intended, the transformer steps the 25V line down to 2.83V, so I was pretty close to 10:1. I knew it wasn't ideal, but it got us through the game. (And the transformer introduced MUCH less distortion than I ever would have imagined!)

Hope you find a comfortable solution!

Best Regards,
Jim Thorn

Hi Jim,

No, the Mic receiver is Only connected to the left channel. The right channel could be used in lieu of the left, but I only use the right channel if there is a need for two mics.

Thanks,

Bill

 

[Brad Weber]

Re: Amp kicks out on Mic ONLY with 8 ohm to 70 V Xfmr on Amp output

I have used the trick of wiring between the left and right + but agree with the others that this is used for a 'poor man's' surround as it results in reproducing the difference between the two channels rather than the sum of them.

The phono input is intended specifically for that use. In fact you are may be looking at things backwards as a phono input on a receiver like that would usually be intended to be connected direct to a phono source with there then being an internal RIAA preamp, so the last thing you would want to add as an additional external preamp. What you really need is an appropriate input or an external mic preamp and line level input.

I have a feeling that since you are apparently running the wireless microphone into the phono input, which likely has an internal RIAA preamp, and then using the difference between the channels for the output that those two factors are causing your problems. This may be a situation of having limited funds but you seem to be trying to 'band aid' a consumer system that is really not appropriate for the use and will probably never be anything other than problematic. This is a pro audio forum and maybe this is a good opportunity to suggest a pro audio solution.