Tandberg 3001 2.0

Ok, just in time for Christmas I am announcing that I plan to do a run of the 2.0 Output boards.

They will include all of the fixes noted in the manual, the biggest being:

• Crossed resistors in fixed outputs
• Multipath jack being ever so slightly off
• One resistor trace being on the wrong side of the board.

Enhancements include:

• Silver plating instead of tin/lead
• Fully soldermased and silk screened
• Thickest available copper thicknesses

Order yours now for $75. Purchasers of the 1.0 boards receive $15 off. Sorry it can't be more, but these things are expensive when you are doing small runs.

Cheers!

What DID I get into?

Well, folks, the power supply is fully operational. I wish I could say the same for the tuner.

So long as I'm holding onto the tuning dial, it works great. As soon as I release it, it forgets what station it was last on, and tunes to the highest frequency on the dial.

Also, the presets seem to be broken.

On the plus side, it sounds really great, when I can hold on. :)

So, based on this, I'm going to take it to my favorite Cambridge repair place, Audio Lab to see what Derek can do for me.

Maybe for Christmas.

RTFM!

Man, if only I had Read the Freaking Manual (R.T.F.M.) a couple of weeks ago I would be listening to the Tandberg instead of griping about upgrading it's power supply.

In the schematic for the Tuning System 2, which is where P801 goes to, pin 1 is clearly an OUTPUT to the power supply. That is, the 5.2 Volts gets generated elsewhere.

I could blame Tandberg, but honestly, had I remembered correctly how a PNP transistor worked, it would have been pretty obvious to me that it was. Fortunately on page 26 there is actually an arrow showing which direction the signal here is going.

What this means is that I really am done with the Power Supply rebuild! Yeay!

With a little luck, next weekend I'll actually be listening to this tuner for the first time!

A fully re-worked power supply with Mills wirewound resistors, and Panasonic FM caps, a slew of upgraded transistors, and a brand new output board with Cardas connectors and Vishay/Dale metal film resistors, this truly is going to be a pretty good piece of gear to own for a few years more. :)

The TDA 3001: Where are all the Parts?

The power supply is back together, and seems to be working. The 5V supply seems to have some sort of remote sense feedback circuit built in. I don't quite get it, but it was driving me mad until I spent some time with a breadboard and tried out different options.

I've finally decided that rather than assume it's broken, I'm just going to put it back into the tuner and see what it does. I need to check the voltages at the 15V and 25V supplies anyway. While cleaning the header pins which connect to the output board I noticed they had ALL come loose. That is, the solder connections were broken on all of the pins. It wasn't falling out, but it would have made a terrible connection. A few minutes with my 100W solder gun and everything was back where it was. I then took out the alcohol and toothbrush and scrubbed the board to get rid of excess flux and any other dirt which might have accumulated while it was waiting in a box.

The one thing I need before I can re-assemble the tuner is a package of insulated quick-disconnect, crimp on spades. A quick trip to Radio Shack, and the tuner should come back in a snap.

Then I'll probably have to worry about the backup battery and the preselect buttons.

You know, I'm almost ready to begin thinking about my next project. After taking apart the Rega EAR headphone amp, I'm thinking I should consider making my own. The one thing the EAR did well was provide discrete output transistors. Unfortunately, their potential was mired by the use of an aging op-amp design, and single ended supply. I am sure making the product as inexpensive as possible played a major part in these decisions. In order to get rid of the output caps you need a double ended supply, and your biasing solution gets more complicated in order to keep the DC offset as low as possible.

So, maybe next I'll make a headphone amp with high quality op-amp front end, with a Class-A discrete output section. I think I may take apart the InnerSound Electrostatic amplifier I have (manuractured by CODA) and try to use their transistors in my design. Yes, it's ridiculous. but that amp sounds soooo good and it's sooo efficient, that at the voltages we need for headphones they may work really well. Instead of 30 devices per output channel I'll use 2 though. But for this semester, this tuner really is all I'm going to be doing. I have a GPA to maintain. :)

Pics will be coming as I begin the re-assmbly process.

An Afternoon with the Tandberg 3001 Supply

Rather than try a bunch of things and have then be wrong, I ordered lots of spare transistors of the one's I could order, and I spent most of the afternoon today looking for the bad one in the 25V supply.

After replacing a host of little transistors I finally found the one that was causing the problems. The supply is 95% done. The critical 15V and 25V supplies are working perfectly. It wasn't a complete waste of time though, a lot of the transistors were being upgraded to 65V parts, which will make this unit much more stable should I ever have to remove it and modify it on the bench again.

Unfortunately, I'm still having serious problems with the two 5V supplies though. Let me share with you my current (haha!) problems. Take a look at the relevant part of the schematic:



To the left of the 33 Ohm/4W resistor is 15 Volts, as it should be. The problem I'm having is that the two transistors you see here should be outputting 5 and 5.2 V respectively. However, there is too much of a voltage drop through R822, leaving me with something like 2.8 V instead of the 5.9 I should expect. The problem seems to be the current draw through Q814(Q813 on the board). The way I see it, in order for the power supply to behave as documented, there should be about 4.1 mA through R827, and about 1.4mA through Q814. Instead, I'm getting 4.8 mA through Q814 alone. What I don't understand is WHY. The parts all seem to be working correctly. If I remove Q813, and replace Q814 with a brand new part, I still have the same problem. I can't get the voltage at the base of Q813 up to 5.9V. And from what I can see, besides actually changing the value of R827 I really don't have a chance. The input voltage remains a rock solid 15.04 volts throughout though.

I've removed the electrolytic, and measured all of the resistors. Everything is measuring what it should.

Suggestions? Is the schematic bogus to start with?

Modding The Rega EAR Headphone Amplifier

Even though I don't think the Rega EAR is worth the purchase new, I decided to put in information for those of you who may already own a Rega EAR and are kind of curious about getting into making your own modifications. This is actually a device that will be simple to mod for a few bucks which can actually yield some serious benefits.

Tools you'll need:

1. Metric Allen wrench set
2. Phillips screwdriver
3. Soldering Iron
4. Solder Wick
5. Solder

You'll need the metric Allen wrench set to remove the front cover. You'll just strip them otherwise, they are on tight.

Take the back plate off. Before you do anything else, measure the space between the top of the board and the case. You'll need this to make sure you don't buy caps which are too tall.

Disclaimer:
If you read instructions on the internet and follow them without accepting responsibility for your actions, or without really knowing what you are doing and hurt yourself, burn down your house, ruin your stereo or go stark raving mad and start attacking your neighbors lawn mower with an ax while naked in the middle of winter, you are a certifiable dufus and should read no further.

If on the other hand you realise that you are taking risks for the functionality of the equipment you are modifying, and your own health and safety, and will take full responsibility for the consequences of your actions and you are not going to try to act like a dufus by blaming the author of this or any other article online, then please continue.

Capacitors
First, a note about how capacitors are measured. There are three mesuarements which are usually important. Capacitance, Voltage and Temperature. Capacitance is the only value which affects how the part works in circuit. Voltage and Temperature affect when the part will fail. If you put a cap in with too small a voltage rating, it will break down very quickly. For this reason, you can increase the voltage and temperature grades on a capacitor relatively safely. On the other hand, going too far over in voltage may cause you to use a part too physically large, which will be more inductive and may not physically fit. For instance, it's ok to replace a 35V Cap with a 50V or 100V cap. There are cases when the manufacturers original parts were rated too low (see my posts on the Tandberg 3001 power supply) in which case a voltage upgrade makes a really good idea. I don't think you'll find this a problem with the Rega.

One thing to be careful of is that electrolytic capacitors have a direction they should be used in or they will fail fast. Observe how they are mounted in the original, look for (+) signs on the silk screen. Make sure the (-) mark on the capacitor goes to the opposite pin.

Take every electrolytic you see, and replace them with Panasonic FM capacitors, available from Digikey. You should leave the values the same except for the the power supply and output caps. The power supply filter caps are the two large caps rated 1,000uF right next to the power supply jack. Buy the largest cap you can fit into their place. If you can fig 1,800uF you should. This isn't just upgrade-itis. This unit has some serious hum, the only way to remove it is going to be to increase the filter capacitance. By putting in low inductance caps, we can also hope to reduce some of the hiss as well.

For the output caps, putting in bigger units will give you better bass, at the expense of increased inductance and less treble. I suggest that since the Pana FM caps are so low in inductance anyway you should upgrade these by 10-20% if they will fit. You should be able to fit a 0.1uF metal poly film cap underneath each output capacitor. Check the case to board spacing first though.

For all other electrolytic caps, match the original uF values. Leave the ceramic and box caps alone.


Fuse
After adding significant capacitance to the power supply you may blow the little fuse near the jack. Ce est le vie. Short the puppy and move on. Ok, no, just kidding. If this problem arises I would seriously do is add a fuse jack to the back of the unit, and wire it to where this thing is. Then try to put the smallest fuse which doesn't fail when you power the unit on. Somewhere around 200mA to 315mA 5x20mm fuse should work.

You can check the on board fuse for failure by testing the AC voltage across it. It should be near 0. If it's the same as the voltage coming from the wall wart, it's blown.

Op-amp
The single chip in the middle of the board towards the back is an NE5532 dual operational amplifier. Rega probably picked this particular op amp because it makes the rest of the circuit incredibly easy. Let me explain. For an op amp, the 5532 has very low output impedance and is able to drive 600 Ohms to +- 10 V easily. This means they can buffer the input and drive the current gain stage directly, without any more parts. In other words, it's the cheapest possible solution they could find and still have a large current gain capability.

Remove it and replace it with a high quality machined IC socket. This single upgrade is going to give you the most amount of fun, because you'll then be able to experiment with a variety of different dual op amps until you find the one that you like best without having to pull out the solder gun each time.

I suggest the first good one to try is an NE5535. It's probably got the closest sound of the NE5532, but with smoother, richer detail, while still being a rather "cold" part. Philips released it as a Class-A biased version of the NE5532, but I don't think they even bothered to use it in their own products very much and it consumed higher power so except for the modifications market it didn't make much of a dent in the market. I believe Walt Jung (Walt, e-mail me if I'm wrong) wrote about this being a marvelous upgrade to the 5532, but I cold be mistaken. Mouser and Digikey no longer carry it but I found some at ICPlus. Keep in mind that the 5535 does not have the current gain capability of the 5532, so if you have very low impedance headphones, this may not be a good choice, however, several Burr Brown

In any event, once you have socketed the op amp you should be able to find a long list or audiophile recommended parts that will fit, including some who make adapter boards so you can use some of the very expensive Burr Brown single op amps in the socket. Being able to hear the effect different op amps have may alone be a reason to mod the EAR. Here is a good resource for op amp replacements: Tangentsoft Op Amp List

Also, take a look at the op amp pins. I believe there are a couple of unused part locations which were meant to be filter caps but are now marked N/C. You can tell because one of the cap pins will connect to pin 4 or pin 8 of the op amp. If this is true, put a 10uF to 22uF Pana FM cap in each. Pin 4 should connect to the capacitors (-) side. Pin 8 to the (+) side.

Final Words
Like I've said, the Rega EAR is not worth purchasing for the sake of upgrading it. If on the other hand you already have it, you want to learn how to do things yourself, and you are happy to spend about $30 in parts in teaching yourself and your ears how different components affect your sound, then you are in an excellent place. Please let me know how your experiments turn out.

Rega EAR Headphone Amplifier, Final Analysis

After taking a close look at the insides (see previous post) and listening to the Rega EAR headphone amplifier for about 3 hours I can finally give you my reader my definitive take on the product.

I tried four different pairs of headphones:

• AKG K 701
• AKG K 240 Studio
• Shure E4C
• Sony MDR-V600
  

and 2 different source components. I compared the EAR to my SUMO Athena non-headphone enabled preamp, as well as to the direct outputs of my Creative Labs Jukebox Zen 2.0. I actually chose the Athena because I knew that it had limited low impedance drive capability. I figured that it would emulate what a poorly designed headphone jack would do to the sound. On the plus side, it's pure class A and has about a 600 Ohm output impedance, which is pretty darn low for a line level amplifier. Still, it should have been blown away by the EAR.

After a few hours of listening, and after looking closely at the insides I can't recommend this product. Either in absolute terms or price / performance wise.

Compared to the Sumo Athena, it adds too little. The only times I could tell it was a better driver for the headphones was with the 29 Ohm E4C's. Compared to the direct outs, it seemed to muddle the sounds and rob bass drums of their midrange strike and impact. With the AKG 701s it seems to get harsh when there are complicated passages with a lot of upper midrange. Overall, it's a noisy piece of gear, which makes sense given the limited filtering done in the power supply. You can't turn it up too much before the hum of the power supply is clearly evident. The Sony MDR-V600s made the noise problem even worse. They are the most sensitive headphones I have and have a lot more treble than the AKGs which made the hiss from the Rega almost unberaable.

My sample even has some weird oscilation going on when you turned the volume up just a bit, which would disapear, but come on, guys. It's not like you don't have anyone in England who can show you how to design a current amplifier stage that won't oscillate. Otherwise I found the sound to be a complete "Why bother?"

My suggestion to would be amp buyers is to give this one a pass, or buy one, then sell the plastic Rega label on the front so you can go through it top to bottom and mod it until it shines. If. Well, never mind. Don't buy it.