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 Post subject: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 5:07 am 
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Location: Atlanta GA
I know I have stumbled across this subject before on this forum but would somebody please point me in the right direction on what is needed in order to build a pre-amp so that I can raise the volume level from my iPod going into the tape input on a vintage radio? The volume level is about half what the FM radio produces. I have tried 4 different vintage German radios and they all have the same issue. I am having to turn up the volume knob about 2 to 3 times as much compared to listening to FM radio.


I assume Mouser is now the only source since radio shack is out of business.

Thanks,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 5:29 am 
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No need for an amp. What you want is simply a step-up audio transformer. I use 1:10 and get superb sound with the input at 1/4 volume, no distortion.
The transformer also isolates your device from the radio, a very good thing.

Now my little secret is that I use the transformer from cheap little 12 volt wallwarts. I've also used a 7.5 volt phone charger. It works well, but 1:10 is plenty of step-up, all you need. Discard the circuit board with diodes. You are only using the transformer.

These are not Audio transformers, so they are wrong, but work well, 60 cycles may be close enough?

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Last edited by westcoastjohn on Feb Wed 21, 2018 5:15 pm, edited 1 time in total.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 1:52 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Yes, the step up transformer is what I meant. I like the idea of scavenging parts from wallwarts but I am not experienced enough to be sure of what to do. So, I will wait for somebody here to chime in on what I can purchase to accomplish this. If I remember correctly it involved two transformers and a couple resistors or capacitors and that is all. And no power is needed as the device provides the power. I will play this most of the time through a vintage German stereo console so fidelity will be important.

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BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 3:51 pm 
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Location: Mountains of Mourne. Ireland.
https://www.radioshack.com/search?q=amplifier

Greg.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 5:25 pm 
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1) Here's the 2 transformer schematic. But if you combine your stereo inputs first, with the resistors, you only need one transformer.

2) The second schematic is still Not Right, but it shows the general idea and gives a possible number to search for.

https://www.radioshack.com/products/rad ... ransformer

3) For isolation, your input goes into the short windings only, and your stepped-up output is taken from the longer winding. There should be no common connection between the radio and your device. Simple.


Attachments:
passive amp.jpg
passive amp.jpg [ 50.12 KiB | Viewed 787 times ]
ipod-adapter.gif
ipod-adapter.gif [ 2.15 KiB | Viewed 787 times ]
Aux Input.jpg
Aux Input.jpg [ 107.82 KiB | Viewed 784 times ]

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 6:04 pm 
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Location: Atlanta GA
The Radio Shack is showing out of stock. Does anybody know where else I can buy these transformers?

BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 7:04 pm 
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Location: Merrick,NY,USA
Yes, I second that. Has anyone found a readily available sub for these Radio shack transformers? I appreciate West Coast John's suggestion but would like something considerably more compact.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 21, 2018 10:01 pm 
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Location: Cleona, PA
I would try using any audio output transformer you have lying around, off an AA5 or similar radio. Use the hand sketch above. connect the input from the iPod to the transformer leads that used to go to the speaker. The other (tube) side of the transformer would then go to your radio input. You could even try a 6-volt filament transformer with the 6 volt side as input and the 120 volt side to the radio input.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Thu 22, 2018 1:54 am 
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I got some from Antique Electronic Supply, Tempe, AZ, ages ago, not found there now, unfortunately. Those were small enough to fit in a film canister. Like when camera film came in canisters. :lol:

I found this link and that would be about $8 USD.

https://qcomponents.ca/McBride-MCT70.ht ... bPEALw_wcB

Take a look at the DigiKey catalog. I'm sure they have what you need there, but I don't have the part number.

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Last edited by westcoastjohn on Feb Thu 22, 2018 5:07 am, edited 3 times in total.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Thu 22, 2018 4:06 am 
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i had the same question a while ago

there are many suggestions in this thread, but at the end is my solution:

http://www.antiqueradios.com/forums/vie ... 6&t=325500

since then, I removed the Lepai (I used it to boost my TV line level to larger bookshelf speakers) and used one of these Radio Shack Amps instead.

this RS amp works just as great as the Lepai.

this setup works great for me, but see my disclaimer below.

steve


Attachments:
Realistic-SA-102-Radio-Shack-Integrated-Stereo-Amplifier-TESTED.jpg
Realistic-SA-102-Radio-Shack-Integrated-Stereo-Amplifier-TESTED.jpg [ 12.77 KiB | Viewed 731 times ]

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Thu 22, 2018 1:01 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Right now I only want to insert a passive transformer to accomplish the job. I do not know enough about what I am doing to trust installing a small amplifier like that Lapai. I would fry something for sure.

So, I clicked the link from westcoast john's reply: https://qcomponents.ca/McBride-MCT70.ht ... bPEALw_wcB how do we know that is exactly the right transformer for what I need here? I do not see any detailed specifications to indicate just what I will be getting. Am I looking for a ratio of 2:1 or 3:1 etc.? Or am I looking for some other specification? I am interested in keeping good audio fidelity since this will play through a good sounding German console and not just an old AM radio. I think if I know more about what exactly to get I can search online myself until I find one a little closer to home.

Thanks,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Fri 23, 2018 2:20 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Anybody here know where to purchase these transformers since Radio Shack is sold out? I can search the internet myself but I need to know what specs to look for.

Thanks,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Fri 23, 2018 7:18 pm 
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Location: Houston Texas
[quote="bluebellyyankee"]Anybody here know where to purchase these transformers since Radio Shack is sold out? I can search the internet myself but I need to know what specs to look for.

Thanks,
BB[/quote


After our radio shacks closed their doors, I found equivalents at mouser.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Fri 23, 2018 7:45 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2400
Location: Lexington, KY USA
The Radio Shack transformer was a poor choice back when you could still buy one down at the corner. For this application you want a higher impedance primary. Look for something like 150 or 600 ohms to 10k or 50k ohms.

It might be prudent to also seek a specified minimum isolation voltage between in and out.

With most transformers, a lower impedance source will improve the frequency response. If the two mixing resistors are at least 10 ohms, they will protect the stereo outputs. Something like 10 ohm or 47 ohm resistors will work better.

Distributors such as Mouser do sell small audio transformers that will work, and even have at least some rated breakdown voltage.

Ted


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 12:37 am 
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Has anyone thought or tried to use a graphic equalizer between the adapter and radio to shape he audio response? I believe them to be the right impedance for this application. Could make up for the lower bass levels with some gain.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 2:08 am 
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Here's the small output transformer that I use with a Sony 50 CD carousel. The stereo cord from the player plugs into the double RCA jacks. I've got 1.5 K R's on this one. The trans # 167-56 493 7947.


Attachments:
Auxinpt3.jpg
Auxinpt3.jpg [ 163.36 KiB | Viewed 632 times ]

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 4:25 am 
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Joined: Feb Mon 13, 2017 10:46 pm
Posts: 158
Location: Atlanta GA
I give up. I am not versed enough with this transformer stuff to find what I need. I have searched my self silly on the internet trying to figure out what to buy. Would somebody be kind enough to post a link to exactly what I need to purchase and where. I do not mind spending a few extra bucks in order to get good fidelity out of this rig.

Thanks,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 5:18 am 
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Location: Mountains of Mourne. Ireland.
Have you Maxed (turned up the volume fully on the ipod)?

Greg.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 5:28 am 
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by using my setup with the lepai or the realistic amplifier, my set pumps like a jukebox.

i believe this setup would work out well for you, but at the end of the day, it is your choice. if your knowledge is limited, it might not be your best solution if you are not comfortable proceeding forward.

i chose not to play the game with transformers.

steve

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 6:19 am 
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I built a transmitter that I had bought from an eBay seller in Japan, and into which I fed the output from an MP3 player. Unfortunately the input signal was just not strong enough, so I built an LM386 amplifier, and adjusting it's volume just right produced excellent clear volume.

I would suggest that it would work easily as well for your application. If you don't want to build the amp yourself, there are scores of Chinese sellers on eBay who will happily sell you an already built unit for just a few bucks and free shipping.

Here's a scan of the MP3 player, transmitter, and LM386 amp;


Attachments:
Meeny.JPG
Meeny.JPG [ 199.95 KiB | Viewed 617 times ]

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